Egner, 88, who had been a resident of the Silver Glen Cooperative, a retirement and assisted-living community in Bellevue, Washington, near Seattle, died there last week, according to a representative of the facility who refused to give her name.
She declined to give any details about the circumstances of his death.
Egner's lawyer, Robert Gibbs, declined to comment, as did officials for the US Justice Department in Washington. But court documents filed in recent days showed that a pretrial conference on his case set for last Friday was canceled, as was a telephone conference that had been scheduled for last Wednesday.
Egner was listed by the Nazi-hunting Simon Wiesenthal Center in Los Angeles as the most wanted Nazi war criminal still known to be residing in the United States.
Efraim Zuroff, the center's Israel director and chief investigator, told Reuters by telephone that Egner was typical of many mid-level and low-level Nazi operatives "who were actively involved in the mass-murder process."
He said that hundreds of such individuals are believed to remain alive around the world, "but the chance of bringing these people to justice is not very high these days. There are very few countries seeking out Nazi war criminals."
An ethnic German born in Yugoslavia, Egner entered the United States in 1960 and became a citizen in 1966. Serbia issued an international arrest warrant for him in April last year and formally requested his extradition in November.
Egner had admitted he belonged to a despised Nazi-run security unit but denied he committed war crimes.
The US Justice Department had asked the federal court to revoke his US citizenship based on evidence of his role in a Nazi mobile execution unit that participated in the mass murder of more than 17,000 Serb civilians, mainly Jews, Roma and political opponents, between 1941 and 1943. He was accused of concealing his Nazi past when he applied for US immigration and naturalization.
A proceeding to consider whether to revoke his citizenship was set to begin on February 22 at US District Court in Seattle.
Unit executed over 11,000 individuals
Court documents say Egner has admitted to volunteering to serve in the Nazi-controlled secret police in German-occupied Serbia and to guarding prisoners as they were sent by his unit from the Semlin concentration camp near Belgrade to an execution and mass-burial site about 10 miles away in Avala.
According to court records, Egner's unit took part in executing over 11,000 individuals, mostly Serbian Jewish men, in the fall of 1941.
The same unit murdered some 6,200 Jewish women and children who had been rounded up and were gassed to death while being transported in a specially equipped van that made daily trips in the spring of 1942 from Semlin to Avala.
Egner stated that he guarded four transports of prisoners in all, including one that ran to the Auschwitz death camp in Poland, court documents allege.
"Whenever the train stopped, Egner and the other guards would get off the train and stand on either side of the car with their guns to ensure that no prisoner escaped. If one had tried to escape, Egner states he guessed he would have shot at them," according to court records.
Egner further admitted to serving as an interpreter during secret police interrogations of political prisoners that sometimes involved severe torture and often ended in the prisoners' execution, the documents state.
After settling in the United States, Egner worked for a hotel chain in the Pacific Northwest as a food and beverage manager before retiring in 1980. He married, spent years living in Portland, Oregon, and came to the Seattle area to live with family after his wife died in 2005.
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