JERUSALEM — A leading Nazi hunter said on Monday that Adolf Eichmann’s top lieutenant, long one of the world’s most wanted fugitives, died at least four years ago in Syria, where he had escaped justice and may have advised the government.

Efraim Zuroff, director of the Simon Wiesenthal Center’s Israel office, said the lieutenant, Alois Brunner, was responsible for the deportation of 128,500 Jews to death camps, and described him as Eichmann’s “right-hand man.” Eichmann, one of the chief architects of the Holocaust, was apprehended, tried and executed by Israel in 1962.

Mr. Brunner was tried in absentia and sentenced to death by France in 1954, and he had been the subject of at least two assassination attempts attributed to the Mossad, the Israeli secret service.

“He was a notorious anti-Semite, sadist, fanatic Nazi,” Mr. Zuroff said in a telephone interview on Monday, after the British newspaper The Sunday Express reported his confirmation of Mr. Brunner’s death. “The only known interview we have with him was to a German newsmagazine in 1985, in which he was asked if he had any regrets, and he said, ‘My only regret is I didn’t murder more Jews.’ ”