Peter Egner of Bellevue was 88 when he died last Wednesday, The Seattle Times reported Tuesday.
Egner was scheduled for trial Feb. 22 for allegedly lying to the government about his World War II activities, the Times reported.
Authorities said Egner was a member of the Einsatzgruppen, which spearheaded German leader Adolf Hitler's attempt to kill Europe's Jews and others the Nazis considered undesirable, the Times reported.
Serbian officials also wanted to extradite Egner to prosecute him as a war criminal who rounded up tens of thousands of Serbs, sending many to die in prison camps and torturing and killing others, the Times reported.
Egner lived in Portland for 40 years before Department of Justice investigators caught up with him, the newspaper said.
Egner, who first insisted he took no part in any war crimes and didn't know what went on in the camps, finally swore in depositions he was a guard in several transports that took people to death camps.
"We feel cheated by Egner's death. This was a complex case, but one where there was going to be a good resolution. Serbia had agreed to take (Egner) and prosecute him for his crimes," said Dr. Efraim Zuroff, Nazi hunter and the Israeli director for the Simon Wiesenthal Center based in Los Angeles.