An 89-year-old Philadelphia man is being held without bail on a German arrest warrant charging him with aiding and abetting the killing of 216,000 Jews while a guard at Auschwitz.
Retired toolmaker Johann "Hans" Breyer appeared frail during a detention hearing on Wednesday in federal court, wearing an olive-green prison jumpsuit and carrying a cane.
A court in Weiden, Germany, issued a warrant for Breyer's arrest the day before, charging him with 158 counts of complicity in the commission of murder.
Each count represents a trainload of Nazi prisoners from Hungary, Germany and Czechoslovakia who were killed at Auschwitz-Birkenau in 1944.
Lawyer Dennis Boyle argued his client was too infirm to be detained pending a hearing on his possible extradition to Germany. He said Breyer had mild dementia and heart issues.
"Mr Breyer is not a threat to anyone," Boyle said. "He's not a flight risk."
But Magistrate Judge Timothy Rice ruled the detention centre was equipped to care for Breyer, who appeared to comprehend questions about the hearing.
A police officer also testified Breyer and his elderly wife grasped what was happening during his arrest outside their home in northeast Philadelphia.
Breyer has been under investigation by prosecutors in the Bavarian town of Weiden, near where he last lived in Germany.
Breyer has admitted he was a guard at Auschwitz in occupied Poland during the second world war but has said he was stationed outside the death camp part of the complex, and had nothing to do with the slaughter of 1.5 million Jews and others in the camp.
Thomas Walther, an ex-federal prosecutor with the office that investigates Nazi crimes in Germany, now represents family members of some of Breyer's alleged victims. He called for a speedy extradition.
"The German court has to find late justice for the crimes of Breyer and for the victims and their sons and daughters as co-plaintiffs," he said. "It is late, but not too late."
The arrest comes after years of failed US efforts to have Breyer stripped of his citizenship.