The attackers took one of the three alive, drove him to a nearby refugee camp and executed him in the streets, witnesses said. The gunmen then dumped the body into their car and drove around the camp, firing in the air from the car windows.

The shootout raised new questions about the ability of Palestinian security forces to restore order in the volatile Gaza Strip, as Israel and the Palestinians try to cement a fragile cease-fire.

The three inmates killed Thursday had been awaiting trial for their involvement in two separate shooting deaths in the Bureij refugee camp. Thursday’s attackers were members of the families of the two victims, police said.

Early Thursday, the gunmen broke into the central jail, firing at the guards. Sounds of heavy shooting could be heard in downtown Gaza for about two hours, witnesses said.

Police said that some of the gunmen were wearing police uniforms when they entered the jail, although they were not actual officers. An investigation was continuing Thursday morning.

The prison attack is the most severe challenge to Palestinian security forces since Palestinian leader Mahmoud Abbas took office a month ago, with a promise of restoring law and order. Abbas has deployed thousands of security forces throughout Gaza, to prevent attacks on Israeli targets and to ensure calm in Palestinian communities.

Abbas was scheduled to travel from his West Bank headquarters to Gaza later Thursday to follow up on this week’s Mideast summit in Egypt, where he and Israeli Prime Minister Ariel Sharon called for an end to violence.