GAZA, Palestinian Territory – Thousands of Hamas supporters waving the movement’s green flag marched through the streets of Gaza Saturday for the funeral of a militant leader and high-profile former prisoner who was mysteriously shot dead, as the group’s leaders pledged retribution.

Senior Hamas member Mazen Faqha was found shot dead at the entrance of his house in Gaza City late Friday. Hamas, the Islamic militant group that rules Gaza, said Faqha was shot four times in the head with a silenced gun and blamed Israel for “assassinating” him, but provided no proof to support the accusation.

Faqha was sentenced to nine terms of life imprisonment for directing suicide bombing attacks against Israelis. He was freed along with more than 1,000 other prisoners as part of an exchange in 2011 that released captive Israeli soldier Gilad Schalit. Faqha was among dozens of West Bank residents Israel deported to Gaza or elsewhere because of the severity of their crimes.

The funeral included many Hamas militants and leaders who vowed revenge for the killing.

Hamas official Mahmoud Zahar called for a crackdown on collaborators, whom Hamas suspects did Israel’s bidding.

“Certainly, they (Israel) have collaborating hands. We will cut these hands and necks,” he said. “Our means of revenge are multiple, our means of deterrence are multiple and we know very well the means of restoring rights.”

Hamas typically executes those convicted of collaborating with Israel. The Israeli military had no comment.