The Simon Wiesenthal Center voiced anger at Germany on Monday after a death camp guard who was third on the Nazi-hunting group's most-wanted list died months before he was due to go on trial.
Samuel Kunz, charged in July for helping murder 430,000 Jews while a guard at the Belzec death camp, died last Thursday aged 89 in his home near Bonn, western Germany, prosecutors said on Monday.
Kunz was third on the group's most-wanted list and Efraim Zuroff, director of the Centre in Jerusalem, expressed his "deep frustration" that he had escaped justice -- and put the blame squarely on Germany.
"The fact that Samuel Kunz lived in Germany unprosecuted for so many decades is the result of a flawed prosecution policy which ignored virtually any Holocaust perpetrator who was not an officer," Zuroff said.
"It was only the recent, long-awaited change in this policy which led to Kunz's indictment and the opportunity to hold him accountable for his crimes."
Kunz had admitted working at the Belzec extermination camp in German-occupied Poland in 1942-1943 and was charged in July with helping murder 430,000 Jews there. He remained free, however.
"It was clear to us all that Jews were killed there and were later burned too. We could smell it every day," Spiegel magazine quoted him as saying in November 2009.
Kunz was also charged over the deaths of another 10 Jews in two separate incidents which also allegedly occurred at Belzec.
"The court was just about to decide on the opening of the trial. It was set to start in Bonn in February," prosecutor Andreas Brendel, head of the central Nazi war crimes investigation unit in nearby Dortmund, told AFP on Monday.
Kunz, an ethnic German born in the Soviet Union taken prisoner of war by the Germans before working as a guard, took German citizenship after the war and settled in the Rhineland.
He had been called as a witness in another Nazi war crimes trial, that of alleged death camp guard John Demjanjuk, 90, a Ukrainian-born former US auto worker deported to Munich from the United States last May.
Kunz, whose flat was raided by police in January, denied being personally involved in killing people, prosecutors said at the time of the raid. He remained a free man even after being charged.
"The only consolation is that he was charged, he was exposed and at least a small measure of justice was achieved," Zuroff told AFP.
Since the Nuremberg trials after the war, where several top Nazi henchmen were sentenced to death, German authorities have examined more than 25,000 cases but the vast majority never came to court.
But now, with many of the suspected war criminals in their 90s, there has been a minor flurry of arrests and court cases in Germany dealing with war-time atrocities, in what Nazi-hunters say is a welcome change of policy.
The most high-profile case is that of Demjanjuk, whose trial began last November on charges of assisting in the murder of 27,900 people while allegedly a guard at the Sobibor death camp. He denies the charges.
Israel has also called on Germany to do more to bring to justice Dutch-born hitman Klaas Carel Faber, who escaped from a Dutch prison to Germany in 1952 and who has been the subject of numerous failed extradition requests.
In March this year, another Dutch-born hitman who also escaped to Germany, Heinrich Boere, was sentenced to life imprisonment by a German court. His lawyers had said they were planning an appeal and Boere, 89, remains free.
In July, former SS soldier Adolf Storms, 90, died before he could be put on trial over the massacre of 58 Jewish forced labourers.