Two men were arrested and others were being sought for the abduction and killing of Vittorio Arrigoni, 36, who was found strangled in an abandoned house on Friday, Hamas officials said.
An extremist group in Gaza aligned with Al-Qaeda had threatened on Thursday to execute Arrigoni unless their leader, detained by Hamas last month, was freed.
"Gaza is safe and I want to assure all visitors to Gaza that they are safe and secure," Ismail Haniyeh, the Hamas prime minister, told a French journalist. "The crime that took place was an isolated incident ... and we will enforce the law against the perpetrators."
Saeb Erekat, an aide to Palestinian President Mahmoud Abbas, whose Fatah faction was driven out of Gaza by Hamas in 2007, called the killing "a dark page in Palestinian history" and appealed for national reconciliation.
Hamas spokesman Fawzi Barhoum denounced the crime as an attempt "to harm international solidarity with besieged Gaza and to damage the image of the Palestinian people."
But there was also a shiver of fear that radicals are bold enough to challenge Hamas over what they consider its lack of religious fervor.
There was clear outrage among ordinary people in Gaza over the cold-blooded killing of the Italian activist who had helped local fishermen and farmers. Arrigoni had lived in Gaza since arriving aboard a humanitarian aid boat that Israel had admitted despite imposing a blockade on the tiny coastal territory.
"Vittorio was here for the Palestinian people, and they killed somebody who was here for them," fellow Gaza activist Silvia Todeschini, also from Italy, told Reuters. "They will not kick us out. We will stay."
Palestinians liaising with Italian diplomats said Arrigoni's body would be repatriated via Israel on Sunday.
