Updated

Palestinian gunmen killed three young sons of a senior Palestinian intelligence officer Monday, pumping dozens of bullets into their car as it passed through a street crowded with schoolchildren in an apparent botched assassination attempt that could ignite widespread factional fighting.

There was no immediate claim of responsibility for the drive-by attack, which left the schoolbags and a small plastic bag with a sandwich covered in blood.

"I have no words. Words stop at the extent of this crime," the boys' father, Baha Balousheh, told The Associated Press. "I am a father who has lost his children."

Senior officials in the intelligence service, which is loyal to the Fatah Party of President Mahmoud Abbas, blamed the Islamic militant group Hamas for the shooting. Balousheh, a Fatah member, was a lead interrogator in a crackdown on Hamas a decade ago.

Hamas denied involvement and denounced the bloodshed.

While dozens have been killed in Gaza's escalating lawlessness since Hamas defeated Fatah in January parliamentary elections, the death of the children was especially shocking and was likely to trigger widespread confrontations at a time Hamas and Fatah were at loggerheads over the creation of a national unity government.

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At midday, thousands of angry Fatah supporters joined a mass funeral procession for the boys that snaked through the streets of Gaza City. Hundreds of Fatah security officers fired their rifles in the air. Fatah activists burned tires, blocked roads and shut down the city's commercial market in protest.

In the attack, gunmen in two vehicles riddled the car carrying the children with some 60 bullets, Palestinian security officials said. Three of Balousheh's sons, ranging in age from 6 to 10, were killed, in addition to an adult, hospital officials said. Balousheh was not in the car.

Four more people were wounded in the attack on Palestine Street, which is lined with nine schools. The attack sent children in the area running for cover, some dropped to the ground, others fled in panic.

"We saw fire coming from one car. We started screaming and children started running," said Fadwa Nablus, 12, who had been walking to school with her 9-year-old brother.

Hours after the attack, black smoke hovered over the schools.

"This is an ugly and inhuman crime perpetrated by a bunch of lowlifes," Abbas said at his West Bank office in Ramallah. "We condemn it vehemently."

During the funeral procession, mourners shouted "God is great" amid the sound of gunfire. Balousheh arrived surrounded by bodyguards, wiping his eyes as he tried not to cry.

The three dead boys, still wearing their school uniforms, were carried by family members at a mosque. One boy had 10 bullet holes in his head, according to an AP reporter who saw the body.

The bodies were then wrapped in Fatah flags and taken to a cemetery. The nearby headquarters of Hamas' militia remained empty during the ceremony. Officials said they wanted to avoid further tensions.

Security officials said they believed the shooting was an attempt to assassinate Balousheh, noting that the car's windows were tinted, blocking a view of the passengers. Gunmen had twice before tried to kill Balousheh.

"Palestinian security has opened an investigation into the incident, which we consider the ugliest" in the growing chaos in Gaza, the officials said in a statement.

The Hamas-controlled Interior Ministry, which is responsible for security, promised a speedy investigation.

However, a senior intelligence officer, speaking on condition of anonymity, said he believed Hamas was behind the attack.

In a statement, the intelligence service stopped just short of accusing Hamas, saying the shooting was the latest in a series of attacks on intelligence officers that the service has blamed on the Islamic group and its allies.

Hamas spokesman Fawzi Barhoum condemned the attack as an "awful, ugly crime against innocent children." He said the assailants were undermining Palestinian interests by creating chaos and confusion.

The shooting came a day after gunmen opened fire on the motorcade of Interior Minister Said Siyam of Hamas. Siyam's car was not hit and there were no injuries.

An Abbas aide, Palestinian negotiator Saeb Erekat, said he was concerned the attack will lead to a large-scale confrontation. "If this continues, it will lead to our worst nightmare, internal fighting," Erekat said.

Earlier this month, Abbas announced that talks on forming a unity government between Hamas and Fatah had broken down. Last weekend, he raised the possibility of calling early elections, drawing angry protests by Hamas, which said he does not have the authority to dissolve the Hamas-dominated parliament.