"I am almost certain that he's no longer alive," Efraim Zuroff, director of the Simon Wiesenthal Centre's Jerusalem office, told AFP.
Zuroff said that according to a German intelligence officer, Alois Brunner "died four years ago in... Damascus," where he had fled seeking refuge decades ago.
The Wiesenthal Centre "could not confirm the information" for certain, he stressed, but given the 1912 birth year of Brunner, the unrepentant "right-hand man" of leading Holocaust architect Adolf Eichmann, he was in any case unlikely to be alive.
After the war Brunner escaped detection by taking on a false identity and worked for two years for the US occupying forces in Germany, before fleeing to Egypt in 1954 and from there to Syria, where he was protected by successive regimes.
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He was pursued by Nazi hunters and survived assassination attempts allegedly by Israel's secret intelligence service, the Mossad.
"In Brunner's case, he got two letter bombs. He lost three fingers, he lost an eye, so I'm sure that didn't contribute to his (good) health," Zuroff said, in a veiled allusion to the Mossad.
"He was responsible for the deportation to the death camps of 128,500 Jews," Zuroff said -- including 47,000 from Austria, 44,000 from Greece, 23,500 from France and 14,000 from Slovakia.
"He was a fanatic anti-Semite, a sadist and a person who was totally dedicated to the mass-murder of European Jewry."
Almost seven decades after World War II, the hunt for Nazi war criminals continues and the Simon Wiesenthal Centre publishes an annual list of those most wanted.
He has been under investigation in Germany.
In October, the Israeli branch of the Centre urged Germany to prosecute alleged members of Nazi death squads, giving it a list of 80 suspects.