BAGHDAD - Five members of a private security detail protecting a U.S. convoy were killed yesterday when their helicopters came under attack and one plummeted to the pavement through a tangle of electrical wires in one of Baghdad's most dangerous neighborhoods.

The entire four-man crew in one helicopter was killed, and the gunner in a second helicopter apparently died when he was struck by gunfire, U.S. officials said. The crash set off a chaotic five-hour battle in which U.S. attack helicopters crisscrossed the skies over Baghdad and fired at least one Hellfire missile into the streets below.

It was unclear whether the first helicopter - nicknamed a Little Bird because it is small and nimble - crashed as the result of gunfire, because it got tangled in the wires, or as it was trying to land because a passenger was wounded.

A U.S. military official said that at least four of the victims had suffered gunshot wounds to the head, raising the prospect that some of them had been shot on the ground.

U.S. ground forces made their way to the crash site to retrieve the bodies and secure the area, U.S. officials said. It was unclear what condition the bodies were in when they were recovered.

The helicopters were operated by Blackwater, the same private security firm that lost four contractors in March 2004 in an ambush in the desert town of Fallujah, their bodies mutilated, set on fire and hung from a bridge. That episode led to a three-week siege of the city by U.S. Marines.

This time, the Blackwater contractors were on a routine protection detail in Baghdad, using their helicopters to monitor an official convoy traveling by road, U.S. and Iraqi officials said. The company has a major contract with the State Department to protect officials traveling outside Baghdad's protected Green Zone.

An Iraqi military officer said that the convoy being escorted also came under attack, but U.S. and Iraqi officials said that no one in the convoy was killed.

Lou Fintor, a spokesman for the State Department, declined to comment on who was in the convoy.

Yesterday's attack came as the helicopters swooped low over the Fadhil neighborhood, an ancient warren of narrow, twisting streets that is home predominantly to Sunni Arabs and has been the site of numerous battles with insurgents in recent months. The crash took place near the Shorja market in central Baghdad, a military officer said.

Other violence swept Baghdad all day, with multiple bombings in Shiite areas leaving at least seven people dead. Security forces recovered 27 bodies around the city, killed execution style.

The U.S. military announced three more troop deaths, the Associated Press reported: a Marine killed Sunday and two soldiers killed Monday.

In two battles yesterday, U.S. forces killed 13 insurgents, the military said. North of Baghdad in Haditha, the forces engaged in a hectic chase as a group of armed men tried to escape by boat across a lake. After those men met other insurgents, they were attacked by a U.S. aircraft, killing seven.

Separately, insurgents fled across rooftops in Baghdad before throwing grenades and firing at U.S. forces that were in pursuit. The Americans returned fire and killed six of the insurgents.