The Nazi-hunting Simon Wiesenthal Centre confirmed Sunday that Laszlo Csatary, accused of complicity in the killings of 15,700 Jews, had been tracked down to the Hungarian capital.
"I confirm that Laszlo Csatary has been identified and found in Budapest," the centre's director Efraim Zuroff told AFP.
Ten months ago an informer had provided information that allowed them to locate Csatary, 97, in Budapest, Zuroff told AFP by phone. They had paid the informer the $25,000 promised for such information, he added.
In September last year, they had passed on their information to the prosecutor's office in Budapest.
A statement released Sunday by the centre said Zuroff had "last week submitted new evidence to the prosecutor in Budapest regarding crimes committed during World War II by its No 1 Most Wanted suspect Laszlo Csatary."
The centre said the evidence "related to Csatary's key role in the deportation of approximately 300 Jews from Kosice to Kamenetz-Podolsk, Ukraine, where almost all were murdered in the summer of 1941."
Budapest's assistant prosecutor general, Jeno Varga, said: "An investigation is under way. The prosecutor's office will study the information received."
But Zuroff said in the Centre's statement: "This new evidence strengthens the already very strong case against Csatary and reinforces our insistence that he be held accountable for his crimes.
"The passage of time in no way diminishes his guilt and old age should not afford protection for Holocaust perpetrators."
Zuroff told AFP that the British tabloid daily The Sun had photographed and filmed Csatary, having acted on the information that the Wiesenthal Center had released last September.
The online edition of the newspapers announced on Sunday it had found and identified Csatary.
When its reporters confronted him on his doorstep, he had denied any crimes and slammed the door in their faces, the paper reported.
This was the fourth time that The Sun had cooperated with the Centre to put pressure on officials who were dragging their feet to bring Nazi fugitives to justice, said Zuroff.
The Wiesenthal Centre has urged Hungarian prosecutors to put Csatary on trial.
They say he served during World War II as a senior Hungarian police officer in the Slovakian city of Kosice, then under Hungarian rule.
He was complicit in the deportations of thousands of Jews from Kosice and its environs to the Auschwitz death camp in the spring of 1944.
Csatary had treated the Jews in the ghetto with cruelty, whipping women and forcing them to dig holes with their bare hands, he added.
In 1943, a Czech court condemned him to death after a trial held in his absence. He had fled to Canada and had worked as an art dealer using a false identity, before being unmasked in 1995 and forced to flee.