WASHINGTON — In 2020, a United States naval engineer and his spouse made the fateful determination to attempt to promote a few of America’s most carefully guarded army secrets and techniques, the expertise behind the nuclear reactors that energy the U.S. submarine fleet.

Then the couple confronted one other vital alternative: To which overseas authorities ought to they attempt to peddle the stolen secrets and techniques?

The engineer appeared to imagine that soliciting American adversaries like Russia or China was, morally, a bridge too far, in accordance with textual content messages launched in courtroom. As a substitute, Jonathan and Diana Toebbe considered a rustic that was wealthy sufficient to purchase the secrets and techniques, not hostile to america and, most significantly, more and more keen to accumulate the very expertise they had been promoting: Brazil.

The identification of the nation approached by the Toebbes has till now remained shielded by federal prosecutors and different authorities officers. However, in accordance with a senior Brazilian official and different folks briefed on the investigation, Mr. Toebbe approached Brazil almost two years in the past with a suggestion of 1000’s of pages of categorized paperwork about nuclear reactors that he had stolen from the U.S. Navy Yard in Washington over the course of a number of years.

The plan backfired nearly as quickly because it started. After Mr. Toebbe despatched a letter providing the secrets and techniques to Brazil’s army intelligence company in April 2020, Brazilian officers handed the letter over to the F.B.I. authorized attaché within the nation.

Then, starting in December 2020, an F.B.I. spy posed as a Brazilian official to win Mr. Toebbe’s belief and persuade him to deposit paperwork in a location chosen by investigators. Mr. Toebbe ultimately agreed to supply paperwork and provided technical help to Brazil’s nuclear-submarine program, utilizing categorized data he had realized from years working for the U.S. Navy.

Mr. and Ms. Toebbe, who lived in Annapolis, Md., had been arrested in October and pleaded responsible to espionage prices final month. He faces as much as 17 and a half years in jail; she faces as much as three.

Brazil has continued to wrestle with its submarine nuclear reactor program, and has approached Russia to hunt a partnership on the nuclear-reactor design, mentioned a Russian army official who, like all of the folks interviewed for this text, spoke on the situation of anonymity due to the categorized materials and delicate diplomacy concerned.

Final month, only a week earlier than Russia invaded Ukraine, President Jair Bolsonaro of Brazil even introduced up the expertise throughout a visit to Moscow.

Mr. Bolsonaro has tried to take care of a optimistic relationship with President Vladimir V. Putin of Russia, even amid his aggressions towards Ukraine. Analysts in Brazil imagine that Mr. Bolsonaro, a former military captain, is partly hoping to maintain the door open for a partnership on the nuclear reactor expertise.

The Brazilian president’s journey to Russia drew criticism from the Biden administration. Requested about Brazil’s efforts to accumulate Russian nuclear reactor expertise, a senior administration official mentioned Tuesday that in search of to accumulate Russian army expertise “is a nasty wager for any nation.”

In some respects, Brazil was an odd alternative for the Toebbes. Whereas Brazil and america have a restricted army relationship, Mr. Toebbe’s outreach got here throughout a interval of a number of the closest Brazil-U.S. relations in a long time, as Mr. Bolsonaro and former President Donald J. Trump strengthened the nations’ alliance.

Whereas the U.S. authorities initially wished to launch the title of the nation the Toebbes had tried to promote the secrets and techniques to, Brazilian officers insisted their cooperation not be publicly disclosed, in accordance with an individual accustomed to the investigation.

The White Home, Justice Division and F.B.I. declined to remark. American officers have repeatedly mentioned the couple had not tried to promote the secrets and techniques to america’ chief adversaries, nor to its closest NATO allies, like France.

In encrypted messages from 2019 recovered by the F.B.I., Mr. Toebbe and Ms. Toebbe mentioned what seem like totally different plans to promote the secrets and techniques. One plan, Mr. Toebbe wrote, was flawed to even take into account. One other plan, presumably to promote to a friendlier nation, was additionally questionable for Mr. Toebbe, however Ms. Toebbe pushed for it.

“It’s not morally defensible both,” Mr. Toebbe wrote, in accordance with a transcript of the courtroom proceedings. “We satisfied ourselves it was advantageous, but it surely actually isn’t both, is it?”

Ms. Toebbe responded: “I’ve no issues in any respect with it. I really feel no loyalty to abstractions.”

Mr. Toebbe’s public defender has mentioned authorities guidelines stop him from answering questions. A lawyer for Ms. Toebbe declined to debate the case forward of her sentencing, presently set for August. She has repeatedly mentioned in courtroom that the federal government has offered chosen messages out of context.

There have been just a few nations that weren’t overtly hostile to america and will make use of the expertise and designs Mr. Toebbe needed to promote. Solely a rustic capable of construct a nuclear reactor and able to make investments billions in a nuclear submarine fleet can be prepared to funnel him the a whole bunch of 1000’s of {dollars} in cryptocurrency that he was in search of.

Brazil started work on creating nuclear submarines in 1978, initially motivated by its rivalry with Argentina. In 2008, underneath the administration of President Luiz Inácio Lula da Silva, Brazil reinvested in an effort to create a nuclear submarine, to raised patrol and defend its unique financial zone within the Atlantic Ocean, a supply of fossil fuels and different assets.

The nation goals to launch its first nuclear-powered submarine in 2029, a part of a $7.2 billion submarine program. Brazil is constructing 4 extra conventional submarines with the assistance of France, but it surely has been making an attempt to develop a fifth submarine powered by a nuclear reactor by itself — a mission it has struggled with.

Consequently, Mr. Toebbe’s experience, on the best way to make nuclear reactors even quieter and tougher to detect, in addition to different design parts of Virginia-class submarines, would have been of monumental worth to Brazil.

Whereas the Brazilian embassy declined to remark, a senior Brazilian official mentioned the nation had cooperated with American investigators due to the 2 nations’ partnership and pleasant relations between Brazil’s intelligence service and the C.I.A.

Had Brazil gotten caught in search of to buy American secrets and techniques, the relations between the 2 nations, together with intelligence sharing, may have been thrown in jeopardy.

As a substitute, Brazilian officers labored with the F.B.I. after Mr. Toebbe was initially hesitant to deposit the categorized data at a prearranged secret location, referred to as a useless drop.

“I’m involved that utilizing a useless drop location your good friend prepares makes me very weak,” Mr. Toebbe wrote, in accordance with courtroom data. “For now, I need to take into account the chance that you’re not the particular person I hope you might be.”

To trick Mr. Toebbe into believing he was talking with a Brazilian official, the spy instructed him to search for a sign positioned in a window in a Brazilian authorities constructing in Washington over Memorial Day weekend final yr. Such an operation may solely have been carried out with the cooperation of Brazilian officers in Washington.

After seeing the signal, Mr. Toebbe agreed to drop a pattern of the nuclear secrets and techniques he stole from the Navy hidden in a peanut butter sandwich in West Virginia, setting off a series of occasions culminating within the October arrest of the couple.

Julian E. Barnes and Adam Goldman reported from Washington, André Spigariol reported from Brasília, Brazil, and Jack Nicas reported from Rio de Janeiro. Ernesto Londoño in Rio de Janeiro contributed reporting.