BERLIN — Prosecutors in Germany said Monday they have shelved their investigation of 17 former German SS soldiers who were part of a unit involved in a Nazi wartime massacre of more than 500 civilians in Italy, because of a lack of evidence.

The decision brings to a close a decade-long investigation of the former members of the 16th SS-Panzergrenadier Division "Reichsfuehrer SS," eight of whom are still alive, on allegations they were involved in the Aug. 12, 1944, killings in the Tuscan village of Sant'Anna di Stazzema.

The SS unit descended upon the village that morning ostensibly to hunt for partisans, but instead rounded up and shot villagers, according to survivors. Others were herded into basements and other enclosed spaces and killed with hand grenades.

Historical documents are not clear on precisely how many people were killed that day, but the most commonly cited number is 560. Many were children, women and elderly people, and the slaughter was one of the worst in a series of atrocities by Nazi troops in central and northern Italy during World War II.

But Stuttgart prosecutors said the evidence against the men — whose names were not released in accordance with German privacy laws — is not sufficient.

Although they were part of the unit involved in the killings, in order to bring charges of murder or accessory to murder against them, there needs to be evidence showing they were directly involved, the prosecutors' office said in a statement. The statute of limitations has expired on all other possible charges.

"Belonging to a Waffen-SS unit that was deployed to Sant'Anna di Stazzema cannot replace the need to prove individual guilt," the prosecutors said. "Rather, for every defendant it must be proven that he took part in the massacre and in which form."

The case of Sant'Anna di Stazzema had lain dormant for years until the 1990s when Italian authorities discovered reports on the killings drawn up by Allied forces at the end of the war, and began their own investigation.

Ten of the suspects were tried there in absentia, and were convicted and sentenced to life in prison in 2005 for taking part in the massacre.

Extradition requests from Italy for some of the men have been unsuccessful, partially because they were under investigation in Germany, and it was not immediately clear what effect the decision to shelve the investigation in Stuttgart would have.

Italian authorities have also asked in some of the cases for the suspects to serve their Italian sentences in Germany, but no decision has yet been made on that, German authorities said.

Following the Stuttgart prosecutors' decision, Italian lawmaker Paolo Corsini said he would ask the Italian government to request the files on the case to see what else could be done "to avoid impunity for the responsible."

"We are pained and disheartened by the news that there will not be a trial..." he said. "However, we are not resigned."