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The killers ignored appeals from residents to spare the men”s lives, said Amjad, who declined to give his full name, fearing reprisal.

Preventive Security is an especially despised target of Hamas because the agency carried out bloody crackdowns against the Islamic group in the 1990s.

Fatah officials said Hamas fatally shot seven fighters outside the Preventive Security building. A doctor at Shifa Hospital said he examined two bodies that had been shot in the head at close range. The officials and the doctor spoke on the condition of anonymity for fear of reprisals.

Hamas spokesman Sami Abu Zuhri denied the reports of execution-style killings.

“Whoever was killed was killed in clashes,” he said.

Militants and civilians looted the compound, hauling out computers, documents, office equipment, furniture and televisions.

Mr. Abbas, for the first time in five days of fierce fighting, ordered his elite presidential guard to strike back. But his forces were crumbling fast under the onslaught by the better-armed and better-disciplined Islamic fighters.

Early this morning, a Hamas activist was fatally shot in Nablus, the first person to be killed in the West Bank after days of violence in Gaza. The Al Aqsa Martyrs Brigades, a violent Fatah offshoot, took responsibility.

Fatah and Hamas have warred sporadically since Hamas took power from Fatah after last year’s elections, but never with such intensity. Hamas reluctantly brought Fatah into the coalition in March to quell earlier violence, but the uneasy partnership began crumbling last month over control of the powerful security forces.

Israel withdrew from Gaza in 2005, leaving the Palestinians responsible for the first time for running their own day-to-day affairs. Israel maintains control of Gaza”s airspace and coastal waters and monitors the Gaza-Egypt border by video link.