The Simon Wiesenthal Centre has expressed doubts about a report that Aribert Heim, one of the most-wanted Nazi criminals, died in Egypt in 1992.
"There's no body, no corpse, no DNA, no grave," Efraim Zuroff, the centre's leading Nazi hunter, told AP agency.
On Wednesday, Germany's ZDF TV reported that Heim died in Cairo, saying it had found his passport and other documents.
Heim is accused of killing hundreds of inmates at a concentration camp where he was a doctor during World War II.
On Thursday, Mr Zuroff said the report about Heim's death raised "more questions than it answers".
"We can't sign off on a story like this because of some semi-plausible explanation," he said.
"Keep in mind these people have a vested interest in being declared dead - it's a perfectly crafted story; that the problem, it's too perfect," Mr Zuroff told the AP.
In its report, ZDF quoted witnesses, including Heim's son, as confirming that Aribert Heim, who was also known as Doctor Death, he died in 1992.
It said it had found a number of Heim's personal documents, including his passport and personal letters, in a hotel room in Cairo where he lived under a pseudonym.
The TV channel, working with the New York Times newspaper, also said Heim had converted to Islam.
Heim was one of the last major Nazi fugitives believed to be still at large.
He is accused of carrying out medical experiments on prisoners of the Mauthausen concentration camp in Austria during World War II.
After the war he lived in West Germany, working as a doctor.He disappeared in 1962 when police opened an investigation into his past.