A German prosecutor told The Associated Press on Friday that he has opened a murder investigation against a key witness in the trial of John Demjanjuk on allegations the man may have been involved in mass killings at the Nazis' Treblinka concentration camp.

The probe is based on statements from former guards that Alex Nagorny, 94, took part in shootings at the camp in occupied Poland in 1941-42, Munich prosecutor Hans-Joachim Lutz said.

In one, obtained by the AP, former guard Ivan Knysh told Soviet authorities in 1948 that he remembered Nagorny from Treblinka, and that "from his statements to me I know that he participated in a shooting of 3,000 prisoners."

Lutz said he is currently waiting on information from Ukraine to determine whether any of the witnesses are still alive to be cross-examined about their original statements.

Like Demjanjuk, Nagorny is a Ukrainian who served as a soldier in the Red Army and was taken prisoner by the Germans.

Prosecutors allege that Demjanjuk agreed to serve the Germans and was trained at the Trawniki SS camp before being sent to work as a camp guard at Sobibor. The 90-year-old Demjanjuk, who is standing trial on 28,060 counts of accessory to murder, denies ever having served the Nazis in any capacity.

Nagorny, however, admits training at Trawniki and serving as a guard in the Nazi camp system.

Last February, Nagorny testified that he served with Demjanjuk at the Flossenbuerg concentration camp in Bavaria, telling the court that he lived with Demjanjuk in a barracks room in Flossenbuerg and then shared an apartment with him in Landshut, Germany, after the war.

But when asked to identify Demjanjuk in the courtroom, he could not.

Nagorny walked over to the bed where Demjanjuk lay and said quickly: "That's definitely not him _ no resemblance."

When asked about Treblinka, Nagorny denied being at the camp.

"I have heard of Treblinka," he told the court. "Why, I don't know."

Reached by telephone at his home in Bavaria for comment on the new investigation, Nagorny said he did not understand what he was being asked and passed the phone to his wife, Maria.

"They should leave him in peace," she said. "He could be in the graveyard tomorrow; he's old."

The statements implicating Nagorny have been known for some time, but there were three different former guards named Nagorny and it was not immediately clear whether the man who testified in the Demjanjuk trial was the same who allegedly served in Treblinka.

But the judge who investigated for the special German prosecutor's office responsible for Nazi war-crimes probes told the AP she had determined he was the correct suspect.

"I'm convinced we are talking about the same person," Judge Kirsten Goetze said.