Germany has launched a war crimes investigation into an 87-year-old man it accuses of serving as an SS guard at the Auschwitz death camp, the Associated Press has learned, following years of failed US justice department efforts to have the man stripped of his American citizenship and deported.

Johann "Hans" Breyer, a retired toolmaker, from Philadelphia, admits he was a guard at Auschwitz during the second world war but told AP he was stationed outside the facility and had nothing to do with the slaughter of some 1.5 million Jews and others behind the gates.

The special German office that investigates Nazi war crimes has recommended that prosecutors charge him with accessory to murder and extradite him to Germany for trial on suspicion of involvement in the killing of at least 344,000 Jews at the Auschwitz-Birkenau death camp in occupied Poland.

The case is being pursued on the same legal theory used to prosecute Ohio car worker John Demjanjuk, who died in March while appealing against his conviction in Germany on charges he served as a guard at the Sobibor death camp, also in occupied Poland.

The conviction was not considered legally binding because Demjanjuk died before his appeals were exhausted. But prosecutors maintain they can still use the same legal argument to pursue Breyer. Authorities in the Bavarian town of Weiden, who have jurisdiction, are trying to determine if the evidence is sufficient for prosecution. Speaking anonymously, a German official working on the case confirmed that Breyer was the target of the inquiry. Breyer acknowledged in an interview that he was in the Waffen SS at Auschwitz but that he never served at the part of the camp responsible for the extermination of Jews.

"I didn't kill anybody, I didn't rape anybody – and I don't even have a traffic ticket here," he told AP. "I didn't do anything wrong."

He said he was aware of what was going on inside the death camp but did not witness it himself. "We could only see the outside, the gates," he said.

For more than a decade, the justice department waged court battles to try to have Breyer deported. They largely revolved around whether Breyer had lied about his Nazi past in applying for immigration or whether he could have citizenship through his American-born mother. That legal saga ended in 2003, with a ruling that allowed him to stay in the United States, mainly on the grounds that he had joined the SS as a minor and could therefore not be held legally responsible.

Breyer testified in a US court that he served as a perimeter guard at Auschwitz I, which was largely for prisoners used as slave labourers. But he denied ever serving in Auschwitz II, better known as Auschwitz-Birkenau, the death camp area where the bulk of the people were killed. He also said he deserted in August 1944 and never returned to the camp, though eventually rejoined his unit fighting outside Berlin in the final weeks of the war.

A US army intelligence file on Breyer, obtained by the AP, calls that statement into question.

In 1951, American military authorities in Germany carried out a background check on Breyer when he first applied for a visa to the US. The file from that investigation lists him as being with a SS Totenkopf, or "Death's Head," battalion in Auschwitz as late as 29 December 1944 – four months after he said he deserted. The army file was obtained by the AP from the National Archives through a freedom of information act request.

The document is significant because judges in 2003 said Breyer's testimony on desertion was part of what convinced them that his service with the Waffen SS after turning 18 might not have been voluntary, further mitigating his wartime responsibility.

Also weighing in Breyer's favour with the judges was his testimony that he refused to have the SS tattoo; he does not have such a mark today or evidence that one was removed.

Kurt Schrimm, the head of the special prosecutors' office in Ludwigsburg, which carried out the Breyer investigation before it was turned over to Weiden prosecutors, said he felt there was sufficient evidence to bring charges against Breyer.

"All of these guards were stationed at times on the ramps [where train transports of prisoners were unloaded], at times at the gas chambers and at times in the towers," he said.

Weiden prosecutors, who were chosen because the office is nearest to where Breyer last lived in Germany, say it could take several months before deciding whether to file charges.

The Breyer case was handled in the US by the justice department's office of special investigations. Eli Rosenbaum, who previously headed the office, would not comment on any details of evidence that had been collected against him, nor say whether American agencies were involved in helping with the German probe. Breyer was born in 1925 in what was then Czechoslovakia to an ethnic German father and an American mother. Slovakia became a separate state in 1939 under the influence of Nazi Germany. In 1942, the Waffen SS embarked on a drive to recruit ethnic Germans there and Breyer joined aged 17.

Called up to duty in 1943, Breyer said he was shipped off the same day to Buchenwald – in Germany – where he was assigned to the Totenkopf.

By treaty, the US can extradite its citizens to Germany. But Breyer said he would fight any attempts to take him away from the US and his wife and family. "I'm an American citizen, just as if I had been born here," he said. "They can't deport me."