BAGHDAD – Four of the five Americans killed when a U.S. security company’s helicopter crashed in a dangerous Sunni neighborhood in central Baghdad were shot execution style in the back the head, Iraqi and U.S. officials said yesterday.

A senior Iraqi military official said a machine gunner downed the helicopter, but a U.S. military official in Washington said there were no indications that the aircraft, owned by Blackwater USA, had been shot out of the sky. Two Sunni insurgent groups, separately, claimed responsibility for the crash.

In Washington, a U.S. defense official said four of the five killed were shot in the back of the head but did not know whether they were still alive when they were shot. The defense official spoke on condition of anonymity because he was not authorized to speak on the record.

The helicopter was shot down after responding to assist a U.S. Embassy ground convoy that came under fire in a Sunni neighborhood in central Baghdad, said a U.S. diplomatic official in Washington.

A second helicopter also was struck, but there were no casualties among its crew, said the diplomatic official, who spoke anonymously because he was not authorized to make statements.

The doomed helicopter swooped into electrical wires before the crash. U.S. officials said it was not clear if gunfire brought the aircraft down or caused its pilot to veer into the wires during evasive manuevers.

The Iraqi official, who also declined to be identified because details had not been made public, said the four were shot in the back of the head while they were on the ground. The crash occurred in an old neighborhood of narrow streets on the east bank of the Tigris River, north of the central city.

In separate fighting yesterday, U.S. and Iraqi troops battled gunmen firing heavy weapons from concrete high-rises in another Sunni insurgent stronghold, on the west bank of the Tigris north of the heavily fortified Green Zone. Iraq’s defense minister said as many as 30 militants were killed and 27 captured.

Apache attack helicopters buzzed past the tall buildings and radio towers along Haifa Street, while several Humvees drove on the tree-lined street below. Gunfire rang in the background as shells fell, according to AP Television News footage.

The U.S. military said the targeted raids were intended to clear the area of militants, dubbing the operation dubbed Tomahawk Strike 11. The clashes were the second major fighting to break out in the area in less than a month.

The military reported separately that an American soldier was killed yesterday in fighting near the city’s center, but officials declined to say whether it was connected to the Haifa Street fighting. The military also said that two Marines were killed Tuesday in western Anbar province.

In the aftermath of Tuesday’s Blackwater helicopter crash, U.S. forces were planning to blow up the wreckage to prevent people from scavenging equipment, the Iraqi official said.