German prosecutors say they've shelved their investigation of a 93-year-old former SS officer suspected of being part of a wartime massacre of civilians in Italy because he's no longer fit to stand trial.

Hamburg prosecutors said Thursday that there was a "high probability" that Gerhard Sommer could have been charged with involvement in 342 murders for participating in the Aug. 12, 1944 slaughter in Sant' Anna di Stazzema, but that he was suffering from dementia and couldn't follow a trial.

Sommer and others had already been found guilty in 2005 by an Italian military court and sentenced to life in prison in absentia.

But he was never extradited because he would have had to consent to it under German law. Hamburg opened its own investigation last year.