Berlin --German authorities are investigating whether a man due to testify at the war-crimes trial of John Demjanjuk may have committed killings himself as a concentration camp guard, a prosecutor said Tuesday.

Kurt Schrimm, who heads the special German prosecutors' office responsible for investigating Nazi-era crimes, said his researchers are trying to determine whether the Alex Nagorny slated to testify is the same Nagorny implicated by witnesses in killing people in Treblinka.

"It is fully open whether this is the same Nagorny, if it is the same person who is the witness," he told The Associated Press. A formal investigation would be launched only after that question is determined.

Demjanjuk's trial on charges of accessory to the murder of 27,900 Jews as a death camp guard began Nov. 30, 2009 in Munich.

The 89-year-old retired autoworker, who was deported from the United States in May, could face up to 15 years in prison if convicted for his alleged activities training as a guard at the Trawniki SS camp, then serving at the Sobibor death camp in occupied Poland.

Court documents say Nagorny, who lives in southern Germany, was interviewed by German officials earlier this year and told them he served with Demjanjuk at the Flossenbuerg concentration camp.

Although Demjanjuk is not accused of any crimes at Flossenbuerg, the testimony could undermine his contention that he never worked as an SS guard in any camp.

However, Demjanjuk's attorneys say Nagorny, who is in his early 90s, routinely contradicts himself in testimony. They argue that more evidence exists against Nagorny than their own client.

Nagorny's telephone number is not listed and it was unclear whether an attorney is representing him.

Schrimm's office already has already opened an investigation into another witness, 89-year-old Samuel Kunz, on allegations that he was involved in war crimes as a camp guard.

Kunz is being called to testify about the Trawniki camp. He does not claim to remember Demjanjuk specifically.

Reached at his home in western Germany, Kunz declined to comment.

"I won't talk with anyone until I appear in court," Kunz said.

He said the allegations against him were "tied together" with his planned testimony in the Demjanjuk trial so he could give no information on them either.

The court has not said when either witness would testify.

Demjanjuk's son questioned why his father is on trial, while the two witnesses are not.

"The documentary evidence of Kunz and Nagorny being involved in hands-on murder is clear cut," John Demjanjuk Jr. said in an e-mail.

"That the Germans would continue to ignore the crimes of their own citizens and instead bring a dying Ukrainian 7,000 miles without a scintilla of direct evidence that he harmed a single person is proof that they are only persecuting my father in an attempt to acquit Germany of their own crimes."

The prosecution argues that Demjanjuk, a Soviet Red Army soldier, volunteered to serve as an SS guard after his capture by the Germans in 1942.

Demjanjuk denies ever having served as a guard, saying that he spent much of the war in Nazi POW camps before joining the so-called Vlasov Army of anti-communist Soviet POWs. That army was formed to fight alongside the Germans against the invading Soviets in the war's final months.

According to documents filed in court by Demjanjuk's attorney Ulrich Busch, both Nagorny and Kunz were ethnic Germans who were captured fighting for the Soviets and volunteered to serve the Nazis. Kunz is listed as having served in the Belzec death camp and elsewhere, while Nagorny is said to have been at Flossenbuerg.

Kunz is accused by witnesses of personally killing prisoners, including shooting at least seven in 1943, according to the documents. Other witnesses say Nagorny was in groups of guards that shot hundreds of prisoners at Treblinka.

Schrimm said he hoped to know how his office would proceed on Nagorny early in the new year.

"The people are old in these cases, so time is tight," he said.