A week into his detention, Astrid Senior, Schmidt’s mother, who lives in Lyndeborough, N.H., said the family was still seeking answers about why her son is being held.

“I’m feeling shaken up,” Senior said, stressing the point that her son was here legally. “It’s unbelievable what’s happening now.”

This case, involving the apprehension of a legal permanent resident with few answers available, is the latest instance of unusual immigration enforcement under the new Trump administration, which has pledged a hard line on immigration. Schmidt’s case is part of a wave of reports involving people with green cards or visas being detained by immigration authorities after returning from trips out of the country.

News of his arrest comes after a Rhode Island physician, Dr. Rasha Alawieh, who had traveled to Lebanon to see her parents, was prevented from re-entering the US at Logan on Thursday despite having a visa running through 2027. It also follows the recent arrest of Mahmoud Khalil, a graduate student and outspoken figure during the Gaza war protests last spring at Columbia University, who was arrested last weekend at the behest of the Trump administration. His detention has generated national headlines and protests from progressives.

In the labyrinth of immigration law, each of these cases has slight but significant differences. Schmidt faced misdemeanor drug and drinking charges years ago and was allegedly flagged by immigration authorities for failing to show up to court in 2022, while Alawieh and Khalil do not appear to have criminal records in the country. Schmidt’s family says he has faced no new legal issues since he was granted a new green card, and that his failure to show up in court wasn’t an issue during the renewal process.

Schmidt and Alawieh’s cases also differ from Khalil’s because they were reentering the country, which put them in vulnerable positions, according to immigration attorney Heather Yountz of the Massachusetts Law Reform Institute. A visa is a request for entry, but does not guarantee it, and people entering on visas can be subject to additional screening. Someone with a green card can typically just present it and enter unless it’s flagged for some sort of violation.

“You are more vulnerable when you’re seeking to enter the United States than you are when you’re in the United States,” she said.

That’s why, she said, the actions against Khalil were shocking: He seemed to be stripped of his green card while in the US because of his political opinions.

“He was in a much more legally secure situation,” she said, noting that the constitutional right to free speech applies to everyone in the US.

People with work visas and green cards are heavily vetted, with background checks and fingerprinting, said Boston-based immigration attorney Annelise Araujo. To get a green card, applicants are checked against all the federal agencies to make sure there’s nothing that would make them inadmissible, which could include specific criminal issues or national-security concerns.

All three of these detentions, she said, are “going to have a chilling effect on who wants to come here, and who wants to study here, and who wants to work on our hospitals.”

US Customs and Border Protection would not provide answers about Schmidt’s case, citing federal privacy concerns. Border protection officers determine admissibility of foreign nationals on a case-by-case basis, according to the law, and treat travelers with professionalism, agency officials said.

“If statutes or visa terms are violated, travelers may be subject to detention and removal,” Hilton Beckham, assistant commissioner of public affairs, said in a statement Saturday.

No charging documents against Schmidt have yet been filed with the Justice Department’s Executive Office of Immigration Review, which operates the immigration courts.

Schmidt, who works as an electrical engineer, was returning from a week-long trip visiting his father in Germany when he was stopped by immigration officials at Logan on March 7, his mother said.

Bhavani Hodgkins, Schmidt’s girlfriend, waited for hours for him to come out of the airport that night so they could return home to Nashua. Hodgkins said she eventually spoke with US Customs and Border Patrol officials who told her Schmidt was being held indefinitely and she should go home.

Hodgkins said she called for updates every day and was eventually told by immigration officials that Schmidt had been sent a letter in 2022 asking him to appear in court, why exactly is unclear. Schmidt never got the letter, Hodgkins said.

And his failure to appear in court was not flagged in 2023 when he applied and received a new green card, after misplacing his previous one, Hodgkins said.

“He was never informed there was problems,” Hodgkins said.

Most green cards are valid for 10 years.

According to his family, Schmidt has struggled with drug and alcohol addiction in the past. He has been charged with misdemeanors, including possession of a controlled substance, driving under the influence, hunting without a license, and failure to appear for jail, while he lived in California, according to court records.

Court records show that Schmidt has attended recovery programs, paid small fines, and was placed on probation to resolve his cases.

There are no current charges against him, Hodgkins said.

“He suffered from his own demons,” she said. But he went into rehab when he moved to New Hampshire in 2022 and has been “dedicated to his own sobriety,” Hodgkins said.

She and Senior said that Schmidt has complained about his treatment in detention. He fell ill while being held by immigration authorities and collapsed, they said.

Hodgkins said Schmidt told her that he was taken to the hospital in handcuffs and tested positive for the flu.

According to Massachusetts State Police, Schmidt was taken to a Boston hospital from the customs area of the international terminal at Logan Airport on Monday morning. Tim McGuirk, a spokesman for the State Police, said CBP officers accompanied him to the hospital “as they were not able to process the individual through customs” beforehand. He referred further questions to customs and border patrol officials.

Customs officials could not provide information Saturday afternoon about why Schmidt was taken to the hospital and where he was held between his scheduled arrival on March 7 and the hospital trip on March 10.

Schmidt was moved to the Wyatt Detention facility on Tuesday, his family said.

Officials with the German consulate in Boston have spoken to Schmidt’s families and authorities.

“The Consulate General of the Federal Republic of Germany in Boston is aware of the case and has been in close contact about it with the relevant US authorities as well as with the family of the concerned person,” a spokesman for the German Embassy in Washington.

“He was not given fair treatment,” Hodgkins said. “This is unjust and very painful for us as a family.”

Deirdre Fernandes can be reached at deirdre.fernandes@globe.com. Follow her @fernandesglobe. Sean Cotter can be reached at sean.cotter@globe.com. Follow him @cotterreporter. Camilo Fonseca can be reached at camilo.fonseca@globe.com. Follow him on X @fonseca_esq and on Instagram @camilo_fonseca.reports.