WASHINGTON — The 89-year-old Johann Breyer shuffled unsteadily into a federal courtroom in Philadelphia on Wednesday morning, using a cane for support as he sank slowly into a chair at the defense table.

The retired toolmaker from what was then Czechoslovakia, who immigrated to the United States in 1952, was thin and pale and dressed in a green uniform after a night spent in jail following his arrest at his home in Philadelphia. He looked confused at times, too, but when the judge asked him if he understood why the German authorities wanted to put him on trial there, he answered simply, “Yes.”

Nothing about his demeanor suggested the long-ago secrets that the authorities in both Germany and the United States say Mr. Breyer has carried with him for 70 years. As an armed guard at the Nazi concentration camp at Auschwitz and a member of the notorious SS “Death’s Head” battalion, the authorities charged on Wednesday, Mr. Breyer was complicit in the gassing of 216,000 Jews taken there in 1944 from Hungary, Czechoslovakia and Germany.

The Germans, seeking to have him extradited to stand trial, have charged him with 158 counts of aiding and abetting in murder — one count for each of the 158 trainloads of Jews taken to the killing center at Auschwitz in a six-month span. Most of the so-called deportees, including many thousands of women, children and old people, were killed in gas chambers almost immediately after arriving at Auschwitz, with their bodies burned in four crematories.

Mr. Breyer acknowledged two decades ago, when first questioned by the American authorities, that he had worked as a guard at Auschwitz, but he said that he had done so involuntarily and had nothing to do with the gassings. His lawyer, Dennis Boyle, insisted Wednesday that Mr. Breyer had worked in a prison section of Auschwitz, not among the guards in the extermination area. “He was absolutely not one of those guards,” Mr. Boyle said.

American and German investigators assert — based on Auschwitz camp rosters and newly disclosed documents — that Mr. Breyer was a willing collaborator who participated in the death-camp operations after first volunteering for SS duty at the age of 17. German prosecutors brought the charges against him in a secret, sealed indictment last year, court records show, but the charges were only made public on Wednesday after his arrest.

Prosecutors do not believe they need to establish that Mr. Breyer “pulled any levers” at Auschwitz, Andrea Foulkes, an assistant United States attorney, told reporters after the hearing on Wednesday, but only that he “made it possible for the killings to happen.”

As part of their routine, the “Death’s Head” guards at Auschwitz were responsible for taking incoming prisoners from the trains for “selection” to the gas chambers and, from their positions at watchtowers and along the camp’s barbed-wire perimeter, for preventing escapes.

While many Nazis lived in the United States for decades with little fear of scrutiny, the authorities began belatedly trying to identify and deport them beginning 35 years ago after demands for action from Congress. Since then, Justice Department prosecutors have brought charges against more than 130 aging Nazi suspects, but none older than Mr. Breyer. He could end up being the last Nazi defendant on American soil.

For years, Germany was unwilling to take back Nazis discovered in the United States, causing tensions between the two countries. But Mr. Breyer’s surprise arrest at his home in Philadelphia on Tuesday night, based on the extradition request from Germany, underscored a new cooperation between the two countries in prosecuting people suspected of having been Nazis.

Since the 2011 conviction in Germany of John Demjanjuk, a onetime autoworker in Ohio who was a guard at the Sobibor concentration camp, the Germans have renewed their efforts and opened investigations of several dozen suspects now in their 80s or older.

Mr. Breyer’s arrest also reignited a longstanding debate: whether any person suspected of having been a Nazi is too old to be prosecuted for past crimes.

Some comments posted Wednesday on social media sites objected to the arrest of an 89-year-old, saying that the time had passed for litigating crimes seven decades old. But others supporting the arrest said that crimes as heinous as genocide deserved to be punished, no matter how long ago they occurred.

“The fact that this guy got away with what he did for so long doesn’t mean that he should continue to get away with it,” said Neal Sher, the former director of the Justice Department’s Nazi-hunting office. “These are unparalleled crimes that have no statute of limitations.”

Mr. Breyer’s arrest revived a long-dormant case. The Justice Department first accused Mr. Breyer of Nazi ties and tried to deport him in 1992. But the prosecutors ultimately lost the case, and he was allowed to stay in the country after a long legal fight that hinged on his claims that he had been a United States citizen since birth because his mother was born here. (She moved to Czechoslovakia as a child.)

Since the collapse of the original deportation case against Mr. Breyer, the American authorities have tried for more than a decade to persuade their German counterparts to seek his extradition and put him on trial for his role at Auschwitz. Only now, with the aggressive new stance from German prosecutors, have those efforts succeeded.

Despite facing the earlier threat of deportation, Mr. Breyer escaped the notoriety of many other immigrants who have been publicly accused of Nazi complicity, and his name has remained unknown even to scholars of Nazi war crimes.

Much of the court hearing on Wednesday focused on whether Mr. Breyer should be freed on bail while the extradition case moved forward.

Mr. Boyle, Mr. Breyer’s lawyer, said his client had early signs of dementia and other health issues. He said that Mr. Breyer posed no flight risk and that his frail health should make him eligible for bail.

But Magistrate Judge Timothy R. Rice refused to release him, citing “the serious nature of the crime.” After a short exchange with the defendant, the judge also said, “It seems that Mr. Breyer does have the ability to understand the nature of the proceedings against him.”

nytimes.com