GAZA — For Vittorio Arrigoni, an Italian pro-Palestinian activist who friends said fought peacefully for justice, the end was as violent as it was incongruous.

Police officers from Hamas, the Islamic militant group that rules Gaza, found his body in a house in Gaza City that was empty of furniture, except for the mattress on which the body was lying, according to witnesses. The doctor who performed the autopsy said Mr. Arrigoni’s killers had used a plastic cord to strangle him.

And after years of championing the Palestinian cause, the 36-year-old Mr. Arrigoni apparently died at the hands of a fringe group of Palestinians, inspired by Al Qaeda, that was seeking the release of a local Islamist leader.

Mr. Arrigoni is the first foreigner to have been kidnapped here since the Hamas takeover of Gaza in 2007, and appears to be the first pro-Palestinian activist in memory to have been killed here by Palestinians.