BAGHDAD, Iraq - 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 today.

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.

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 U.S. official spoke on condition of anonymity because he was not authorized to speak on the record.

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.

It also was not clear whether gunfire actually brought the small helicopter down or caused the craft to drop toward the ground, where it became entangled in electrical wires, the U.S. official said. The helicopter was virtually destroyed and after investigating the site, U.S. forces had been planning to blow up it up to keep people from scavenging the parts, the official said.

Blackwater USA confirmed that five Americans employed by the North Carolina-based company as security professionals were killed, but provided no identities or any details.

Today, U.S. Ambassador Zalmay Khalilzad offered condolences for the five Americans killed.

“We had a very bad day yesterday,” Khalilzad told reporters during a round-table discussion at the embassy in the heavily fortified Green Zone in Baghdad. “We lost five fine men.”

He said he had traveled with the men who were killed and had gone to the morgue to view the bodies.

Khalilzad did not give more details, saying the crash was still under investigation and it was difficult to know exactly what happened because of “the fog of war.”

Another American official in Baghdad, who spoke on condition of anonymity, said three Blackwater helicopters were involved. One had landed for an unknown reason and one of the Blackwater employees was shot at that point, he said. That helicopter apparently was able to take off but a second one then crashed in the same area, he added without explaining the involvement of the third helicopter.

The New York Times, citing unnamed American officials, reported that the helicopter's four-man crew was killed along with a gunner on a second Blackwater helicopter.

The Qatar-based Al-Jazeera television said the 1920 Revolution Brigades insurgent group claimed responsibility for shooting down the helicopter and showed a video taken by a cell phone of a mass of still-smoldering twisted metal that it was said was the wreckage of the chopper.

Another Sunni insurgent group, the Ansar al-Sunnah Army, also claimed responsibility and posted identity cards of men who were on the helicopter on a Web site, including at least two that bore the name of Arthur Laguna, who was later identified by his mother as among those killed.

Laguna was a 52-year-old pilot for Blackwater who previously served in the Army and the California National Guard, his mother, Lydia Laguna, of Rio Linda, Calif., told the AP. She said she received a call from her other son, also a Blackwater pilot in Baghdad, notifying her of Arthur's death.

Witnesses in the Fadhil neighborhood told the AP that they saw the helicopter go down after gunmen on the ground opened fire. Accounts varied, but all were consistent that at least one person operating the aircraft had been shot and badly hurt before the crash.

The helicopter was believed to have been escorting a VIP ground convoy as it headed away from the heavily fortified Green Zone.

A report in the Washington Post, also citing unnamed U.S. officials, said one of the Blackwater victims was killed as he traveled with the convoy on the ground.

Blackwater USA provides security for State Department officials in Iraq, trains military units from around the world, and works for corporate clients.

“These untimely deaths are a reminder of the extraordinary circumstances under which our professionals voluntarily serve to bring freedom and democracy to the Iraqi people,” the Blackwater statement said.

Katy Helvenston, mother of Scott Helvenston, a Blackwater employee who died in March 2004, said Tuesday's crash “just breaks my heart.”

Before Tuesday's crash, at least 22 employees of Blackwater Security Consulting or Blackwater USA had died in Iraq as a result of war related violence, according to the Web site iCasualties.org, which tracks foreign troop fatalities in Iraq.