A commander of an Iranian-backed militia has been charged with plotting to attack Jewish sites in the United States, including a synagogue in New York City, and carrying out terror attacks in Europe as part of a broader campaign of retaliation by Iran since the war began in February.

A criminal complaint unsealed on Friday accused the commander, Mohammad Baqer Saad Dawood al-Saadi, of planning at least 20 attacks against U.S. and Israeli interests in Europe and Canada since late February. Mr. al-Saadi was detained in Turkey recently and handed over to U.S. authorities, Mr. al-Saadi’s lawyer said in federal court in Manhattan on Friday. Mr. al-Saadi appeared in court on Friday, but did not enter a plea.

Mr. al-Saadi, according to the complaint, is a commander of Kataib Hezbollah, an Iraqi militia that is a proxy for the Islamic Revolutionary Guards Corps and has helped Tehran project power across the region, including through attacks on American forces and diplomatic targets. While Kataib Hezbollah has long been one of the most important groups serving as an armed proxy for Iran in the Middle East, it does not have a history of organizing attacks outside the region.

Since the U.S. and Israeli militaries attacked Iran in February, the authorities in Europe and the United States have heightened security, especially at Jewish sites, warning of retaliation. The Iranian government’s forces have carried out attacks on U.S. military sites in the Middle East and targeted infrastructure in Arab nations closely aligned to the United States.

The complaint directly tied Mr. al-Saadi, 32, to an obscure group called Harakat Ashab al-Yamin al-Islamiyya that emerged in March. The group took credit for attacks in London, Belgium and the Netherlands without disclosing its links to the Iraqi militia.

At the time, counterterrorism officials said they were investigating whether the group had ties to Iran and whether the attacks were part of a wave of low-cost, unsophisticated methods to sow fear in Jewish communities across Europe. The people accused of carrying out the European attacks do not appear to be linked to extremist groups and were most likely recruited with promises of money, according to authorities and lawyers.

But U.S. authorities, according to the complaint, say Harakat Ashab al-Yamin al-Islamiyya was a front for Kataib Hezbollah, a U.S. designated terrorist group, and investigators noted the similarities between their logos.

“Essentially overnight, Harakat Ashab al-Yamin al-Islamiya was able to activate terrorist cells across Europe” in response to the war in Iran, according to the complaint, which was signed by a special agent of the Federal Bureau of Investigation. The violence stoked a new wave of anxiety for European Jews who were fearful that their communities would be targeted.

The arrest came as President Trump weighed renewing strikes to force Iran to meet his demands. Since late February, the strikes have focused on killing Iran’s top leaders, including the ayatollah, and disrupting the government’s abilities to operate terrorist networks across the region.

Since Mr. Trump’s first term in office, the Department of Justice has said it has disrupted numerous plots linked to Iran, including at least one aimed at killing the president, but not directly organized or carried out by Iran or its proxies.

From its inception, the Kataib Hezbollah has been closely tied to Iran’s Quds Force — the overseas arm of the powerful Revolutionary Guards. It made evicting U.S. forces from Iraq a primary focus. Kataib Hezbollah’s repeated attacks on U.S. Army posts in Iraq and Syria over the years contributed to Washington’s decision in 2009 to designate it as a foreign terrorist organization.

The complaint says that Mr. al-Saadi planned to kill “Americans and Jews” in Los Angeles and Arizona and that he had started planning an attack on a synagogue in New York City.

Mr. al-Saadi is one of the highest level figures tied to Iran known to have been arrested by the United States since the start of the war. For years, and during the current conflict, the United States and Israel have focused on killing Iranian officials.

As a leader of Kataib Hezbollah, Mr. al-Saadi worked with Maj. Gen. Qassim Suleimani, the head of Iran’s security machinery, according to the complaint. The U.S. military killed Mr. Suleimani in a drone strike in 2020.

Mr. al-Saadi’s case appears to involve the kind of retaliatory act of terrorism that U.S. officials have long anticipated and feared.

Jay Clayton, the U.S. attorney in Manhattan, said, “Al-Saadi attempted to disrupt American society through intimidation and violence. In a righteous and just contrast, his prosecution will highlight the best of our country.”

Speaking at a Friday night prayer service at Temple Emanu-El on Manhattan’s Upper East Side, one of the country’s largest Reform synagogues, the New York police commissioner, Jessica S. Tisch, said the plot in New York had intended to target “the heart of our Jewish community.”

She did not identify the synagogue, but said that Mr. al-Saadi had chosen it because of its support for Israel. Officials from her department were working with the leaders of the synagogue to ensure its safety, she said.

Since March 9, the complaint says, there have been attacks against synagogues in Liège, Belgium and Rotterdam, Netherlands, and against a Jewish school in Amsterdam. There have been attacks against the Bank of New York Mellon in Amsterdam and the Bank of America building in Paris, which are both U.S. based companies.

In London, there was an arson attack on four Hatzalah ambulances, which serve primarily Orthodox Jewish communities, and in late April in that city, an individual stabbed and seriously injured two Jewish men — including a dual U.S.-British citizen.

The complaint says Mr. al-Saadi worked closely and in person with Mr. Suleimani, who led the Quds Force before his death in the drone strike. Mr. al-Saadi also worked closely with the Iraqi militant leader who led Kataib Hezbollah, known by the nom de guerre Abu Mahdi al-Muhandis, who was also killed in the same drone strike, the complaint says.

Mr. al-Saadi, in a March 20 phone call, discussed with an individual the European terrorist attacks and said he sought help in planning additional attacks in the United States and that he was willing to kill people in them, according to the complaint. He referred to “Ashab al-Yamin” on the call, but said they did not need help in Europe. “Things are working for us here in Europe,” he said.

The complaint describes the person Mr. al-Saadi spoke with only as “a source of information for the F.B.I.” — or SOI-1, who recorded the call and has voluntarily provided information to the bureau about Mr. al-Saadi.

The complaint says Mr. al-Saadi claimed he was “running multiple teams” and asked whether SOI-1 could help carry out attacks in the United States and in Canada, against targets like “Jewish synagogues” and “Zionist headquarters.”

In an April 1 call, Mr. al-Saadi asked SOI-1 for someone who could carry out such attacks and how much that person would want to be paid, the complaint says. SOI-1 said he was able to find “a Mexican person” who was willing to carry out a bombing operation.

According to the complaint, Mr. al-Saadi said during the call: “This war will not end. Either they eradicate us, or we eradicate them.”

In a call two days later, SOI-1 introduced Mr. al-Saadi to an undercover law enforcement officer who was posing as a Mexican cartel member who SOI-1 said could carry out a terrorist attack in New York and elsewhere in the United States.

That same day, Mr. al-Saadi texted the purported cartel member a photograph and a map showing the location of a prominent Jewish synagogue in Manhattan and an Arabic document describing its congregation as one supporting “the right for Israel to exist,” the complaint says.

Mr. al-Saadi also sent photos and maps of additional potential targets: two prominent Jewish centers in Los Angeles and Scottsdale, Ariz.

The complaint says Mr. al-Saadi agreed to pay $10,000 for the attacks, and on April 4, he paid the purported cartel member $3,000 in cryptocurrency in anticipation of the synagogue attack in New York.

The complaint says that on April 6, Mr. al-Saadi relayed a message to the purported cartel member through SOI-1 that “the operation needs to happen TODAY.”

That evening, the purported cartel member sent Mr. al-Saadi a video showing there was a police presence on the block where the synagogue was located. About an hour later, Mr. al-Saadi asked for an update on the status of the attack, but did not receive a response, the complaint says.

The next morning, it says, Mr. al-Saadi again asked the purported cartel member what had happened, and why the operation had not been completed.

Mr. al-Saadi was charged with, among other counts, conspiring to provide material support to Kataib Hezbollah and the Revolutionary Guards and conspiracy to bomb a place of public use.

On Friday, Mr. al-Saadi entered the courtroom shortly after 1 p.m. wearing army green pants and a black shirt. He nodded as a magistrate judge, Sarah Netburn, read the charges, and he then spoke animatedly with his lawyer through an interpreter. The lawyer, Andrew Dalack, told the judge that his client understood the charges and wanted the court to know that he was a political prisoner.

As the hearing came to a close, Mr. Dalack said his client wanted him to reiterate that it was “very important” for the court know that “he is a political prisoner and prisoner of war and should be treated as such.”

Euan Ward and Adam Goldman contributed reporting.

Good morning. It’s Friday. We’ll look at the Manhattan congressional campaign of Jack Schlossberg, President John F. Kennedy’s grandson. We’ll also get details on an obscure metric that will figure in the new second-home tax in New York City.

Jack Schlossberg is one of nine candidates running for a House seat in Manhattan. When he entered the race, he was known for his social media personality — and for being President John F. Kennedy’s grandson. But rapid staff turnover and Schlossberg’s own erratic actions have gotten attention in the New York political world. With the Democratic primary only six weeks away, I asked my colleague Nicholas Fandos for an assessment.

His campaign sounds like it’s not ready for prime time. Is that a reasonable assessment? This is the first time he’s run for office.

There are few political campaigns that I’ve covered that are not at least a little messy. It comes with the territory. But the degree of the messiness in Schlossberg’s campaign, and the specifics, are unusual.

He has had a surprisingly high rate of staff turnover — at least two campaign managers, two field directors, a handful of advisers and a rotating cast of consultants in the six months since he announced his candidacy.

On the day of his campaign launch, Schlossberg said he needed to nap and then was unreachable for most of the rest of the day, which left his team scrambling. A couple of weeks after the announcement, he had a run-in with a sitting congressman who felt that Schlossberg had ripped off his social media commentary.

Many candidates run on their records. What does Schlossberg list as accomplishments when he’s asked?

Schlossberg has relatively little professional experience, so when he talks about what qualifies him for the job, he brings up things like receiving law and business degrees at Harvard, scoring near the top on the bar exam and being an E.M.T. in college. He’s also worked as a freelancer for Vogue and as an assistant at the State Department, but both of those gigs lasted only a few months. And he readily points out that he has built a large social media following.

There is no standard set of qualifications to be a member of Congress, but most people who run have worked their way up through local government or careers in business or the law. Sclossberg has not followed any of those paths.

What was that social media dust-up about?

Social media seems to be one of Schlossberg’s biggest strengths, but it’s also potentially a vulnerability. He has built a large audience with revealing, funny and sometimes outright bizarre posts that are captivating to some and alienating to others.

I found that in at least one case in this campaign, he essentially mimicked a video from Representative Seth Moulton, a Democrat from Massachusetts, commenting on President Trump’s policy on Venezuela. He made his own video, saying almost the same thing Moulton had said. He rewrote the words, but it was still very close.

Moulton’s team was so taken aback that they reached out to Schlossberg, asking, What gives? They were told that he had indeed copied Moulton’s post because he had liked it so much. This may be common social media practice, but it’s traditionally been a no-no in politics.

What about fund-raising? Has he raised more than his opponents?

He’s doing well on fund-raising. He’s not at the front of the pack, but he has raised $2.3 million from donors.

Some of that has come from small donations across the country, and others from large checks from friends of his famous family. He has his own substantial wealth but has said that he does not intend to spend any of it on the race.

Is he the front-runner right now?

There has not been good polling. The internal polls that we have seen show that he’s a slight favorite in a crowded field that includes two state assemblymen, Micah Lasher and Alex Bores. Lasher is a protégé of Representative Jerrold Nadler, who’s vacating the seat they’re all running for.

Two of the other candidates are George Conway, a former Republican who has become one of President Trump’s biggest critics, and Nina Schwalbe, a global health expert.

Schlossberg did not talk to you when you were reporting the story you wrote. What did his aides tell you about Schlossberg’s management of his campaign and the turnover of his staff?

Schlossberg’s campaign did not dispute most of the details in my story. They downplayed the significance of the staff turnover, saying no one should be surprised that a first-time candidate, especially one as intense as he is, would cycle through staff members in a high-pressure campaign.

In addressing his occasional absences from the campaign, they pointed out that the campaign had coincided with the death of his sister Tatiana Schlossberg, from cancer.

Weather

Sunny skies with a high around 67 degrees. Tonight will be mostly clear with a low near 54.

ALTERNATE-SIDE PARKING

In effect until May 22 (Shavuot).

QUOTE OF THE DAY

“I’m standing behind a mylar curtain, building the character. I part the curtain, stand in a fixed position, and allow the audience to feed me with shouts of joy and respect and admiration and whatever else they’re thinking. Let the people have what they came for. Apotheosis.” — André De Shields, who plays the godlike Old Deuteronomy in “Cats: The Jellicle Ball,” on how he gets into character.

What’s the “market value” of your co-op or condo?

Those two words are in quotation marks for a reason. In New York City, the “market value” of an apartment may not be what it sells for, because “market value” is a bureaucratic metric that often underestimates an apartment’s actual worth.

“Market value” will figure in the new tax surcharge on part-time residents of the city. As proposed by Gov. Kathy Hochul, the second-home tax plan would initially target co-ops and condos that have a “market value” of at least $1 million. They would be taxed an extra 4 percent to 6.5 percent in addition to their existing property taxes.

How many apartments would be affected is unclear. The governor’s office says that an apartment with a “market value” of $1 million would sell for about $5 million. But the disparity is often more pronounced: One Midtown Manhattan penthouse with a “market value” of about $4.2 million sold for more than $135 million last year.

The surcharge has been the talk of the New York political world since Hochul announced it last month. It wouldn’t bring in as much revenue as some of its boosters had wanted, but Mayor Zohran Mamdani cast it as an example of how government was making good on his promise to tax the rich.

The “market value” metric would govern second-home surcharge payments for only the first two years of the new tax. After that, the city and the state would rely on a different measurement to determine which properties are above the threshold. That new metric would be added to a tax system that many taxpayers already find impenetrable.

“I know we just sent people around the moon and back, so you’d think anything is possible,” said Jason Haber, a real estate broker and a co-founder of the American Real Estate Association. “But because of how the city tax system is set up, this is crazy complicated in the first place. And they tried to rush it though, I think without fully appreciating its complexity.”

METROPOLITAN diary

Out of stock

Dear Diary:

It was a Saturday, and I was on Fifth Avenue and 14th Street. Two young women were walking and talking behind me.

“Is there anything you need at the market?” one said.

“The will to live,” the other replied.

I couldn’t help myself.

“I don’t think they sell that there,” I said.

We all laughed and kept going.

— Nancy Lane

Illustrated by Agnes Lee. Tell us your New York story here and read more Metropolitan Diary here.

Where is Representative Thomas Kean Jr.?

Lately, curious political watchers have noted where Mr. Kean is not. He seems to be absent on Capitol Hill, where he represents the people of New Jersey’s Seventh District but hasn’t cast a vote in more than two months. Mr. Kean, a Republican, has not been seen on the campaign trail as he runs for re-election in a competitive midterm race.

He has not appeared in pixel form, either: No candid photos have emerged of the incumbent to allay the worries of donors and constituents. Two weeks ago his office released a written statement, attributed to Mr. Kean, explaining that he was dealing with a “medical issue” and would be back “very soon.”

Since then? No reported sightings.

Mr. Kean’s social media accounts have remained active in his prolonged absence. The posts are often written in the first person, but paired with older photos.

“What we are being assured is that his team is carrying the torch,” Tina Shah, who is vying for the chance to unseat Mr. Kean, said on Tuesday at a debate with three other Democrats. “But we elected Tom Kean Jr., not his team.”

Members of Mr. Kean’s team were seated conspicuously in the third row, watching as the Democrats took turns weighing in on their boss’s mysterious absence. Afterward, his chief of staff, Dan Scharfenberger, reiterated a now familiar explanation.

“He’s dealing with a personal health condition, and he’ll be back soon,” Mr. Scharfenberger said in an interview.

When pressed on the nature of the health condition and why Mr. Kean had not been seen in New Jersey or in Washington, Mr. Scharfenberger said, “There’s no cameras where Tom is.”

He declined to elaborate.

National Democratic officials see Mr. Kean’s district — a largely affluent and suburban region that stretches like a girdle across North Jersey from its eastern shoreline to its western border with Pennsylvania — as one of the party’s best pickup opportunities in November. And Democrats had been aggressively targeting his seat well before Mr. Kean began missing votes.

“He’s going to be totally fine, and he’ll be back to a full schedule soon,” Harrison Neely, Mr. Kean’s spokesman, said on Tuesday.

Soon may not be imminent. The Morris County Chamber of Commerce has amended the program for a May 28 breakfast meeting focused on Washington policies, saying that Mr. Kean will no longer be speaking at the event.

“As you no doubt have heard, Congressman Kean will be unable to appear due to his ongoing health concerns,” a notice posted on the chamber’s website states. “We wish him well.”

Mr. Neely had no immediate response on whether the congressman had canceled other similar events.

Democrats vying for the Seventh District seat have seemed cautious about attacking Mr. Kean, perhaps aware of the political risk of going after a man whose condition is not known. But at Tuesday’s debate, a few of them offered some measured criticism about the Republican’s lack of transparency.

“Look, at the end of the day you’re a public servant,” said Brian Varela, who owns child care centers and is campaigning to abolish the Immigration and Customs Enforcement agency. “And while I don’t think you need to be putting your own personal medical history out there, you at the very least have a responsibility to be communicating with your constituents and with your district.”

Michael Roth, who helped to lead the Small Business Administration during the Biden administration, said most workers would be required to be more transparent with their employers if they needed to take two months off.

“If you were missing work, you would tell your boss,” he said. “And Tom Kean Jr.’s boss is the people.”

Rebecca Bennett, a former Navy helicopter pilot who has worked as a strategist for health care companies, avoided discussing Mr. Kean’s absence and instead said the election was about the job Mr. Kean had been doing before he disappeared from public view.

Mr. Kean is the only Republican on the June 2 primary ballot. If he had exited the race at least 62 days before the primary, a three-person team of campaign advisers would have filled the vacancy. Republican leaders from the district’s six counties would select a nominee if he were to drop out after the primary. His aides maintain that the congressman has no plans to bow out.

Leonard Lance, a moderate Republican who lost re-election in the Seventh District in 2018 after five terms in Congress, said the loss of any of the state’s three G.O.P. House members would be a blow to New Jersey’s historically centrist core. But he said he believed Mr. Kean was still the best candidate “by far” to fend off what is likely to be a fierce Democratic challenge.

“I take at face value the statement of his office that he will be back at 100 percent,” Mr. Lance said.

There were no reported sightings of Mr. Kean on Wednesday, the day after the debate. However, the congressman’s X account did post an update on his activities.

“I’m pleased to join the Congressional Crypto Caucus,” the post said. “The United States must lead on digital assets — driving innovation, delivering regulatory clarity and protecting consumers. I look forward to working with this bipartisan group of colleagues to advance that agenda.”

Kirsten Noyes contributed research.