Hamas officials say the body of an Italian pro-Palestinian activist kidnapped a few hours earlier has been found.
The officials said Hamas police stormed an apartment in Gaza City belonging to a member of the group that released a video of the activist.
After a clash early on Friday, Hamas police found the body, they said. It was not clear how he died. A police officer said four people were arrested in another location in connection with the abduction.
Foreign aid workers in the enclave named the man as Vittorio Arrigoni and said he was an activist with a pro-Palestinian group called the International Solidarity Movement (ISM), who was also working as a journalist and writer.
In a video posted on YouTube on Thursday, the group said it had taken him hostage in order to secure the release of an unspecified number of their members who had been arrested by the security forces in the Hamas-ruled Gaza Strip.
It said it would execute him if their demands were not met by 5:00 pm (1400 GMT) on Friday.
"We kidnapped the Italian prisoner Vittorio and we call on the Haniya government ... to release all our prisoners," it said, referring to Hamas premier Ismail Haniya.
"If you don't respond quickly to our demands, within 30 hours from 11:00 am (0800 GMT) on April 14, we will execute this prisoner," it said.
Arrigoni was shown blindfolded in the 3-minute clip with blood around his right eye and a hand can be seen pulling his head up by his hair to face the camera.
The Arabic text that accompanied the footage of Arrigoni also said "the Italian hostage entered our land only to spread corruption" and it described Italy as "the infidel state".
Arrigoni is the first foreign national to be abducted in the Gaza Strip since BBC journalist Alan Johnston, who was held for 114 days by a group named the Army of Islam. He was released in 2007.