Gunmen in the Gaza Strip on Friday shot dead a Hamas official who was freed by Israel in a 2011 prisoner swap, the interior ministry in the Palestinian enclave said.
Mazen Faqha was released along with more than 1,000 other Palestinians in exchange for Gilad Shalit, an Israeli soldier Hamas had detained for five years.
Iyad al-Bozum, an interior ministry spokesman in the Hamas-ruled Gaza Strip, said that gunmen opened fire on Faqha in the Tell al-Hama neighbourhood.
"An investigation has been launched," he said, giving no further details.
Police spokesman Ayman al-Batniji said Faqha had "four bullets in his head" and said Israel and its "collaborators" were responsible for the killing.
"We know how to respond to this crime," he added.
Faqha was a senior Hamas official in the Israeli-occupied West Bank but after his release Israel transferred him to Gaza.
The Israeli army refused to comment. But Khalil al-Haya, a senior Hamas official in Gaza, said: "This assassination does not serve anybody but the occupiers (Israel), it is of no interest to the other parties."