Dan Senor, a spokesman for the U.S.-led coalition in Iraq, says the U.S. government is committed to a thorough investigation of the events behind the brutal killing.

The White House has condemned Mr. Berg's murder and vowed that his killers will be brought to justice. A spokesman says President Bush extends his deepest sympathy to the family of Mr. Berg, a civilian who went to Iraq seeking work as a contractor.

Video of beheading was posted on an Islamist website

The videotape of the beheading appeared on the Internet Tuesday. Five masked men are seen standing behind a bound and seated Mr. Berg. One of the captors reads a statement in Arabic, saying the killing is in retaliation for the abuse of Iraqi prisoners in U.S. custody. The masked men then push their captive to the ground, and one cuts off his head and holds it up for the camera.

White House spokesman Scott McClellan rejected that link Wednesday, saying terrorists will seek any excuse to justify murder and chaos.

The web site that posted the video says the killing was carried out by Abu Musab al-Zarqawi, who is believed to be part of the al-Qaida terrorist network. That claim has not been authenticated.

Family members say Nick Berg had planned to leave Iraq at the end of March but was held up when he was detained by Iraqi police in Mosul and held for nearly two weeks. Mr. Berg's father has accused U.S. officials of keeping his son in custody, saying that if he was released earlier he might still be alive.

The coalition spokesman said Wednesday that Mr. Berg was never in U.S. custody. Dan Senor says that while in Iraqi detention, Mr. Berg received three visits from FBI agents, who determined he was not involved in any criminal or terrorist activities.

Mr. Berg had gone to Iraq seeking work fixing radio towers. But in two separate trips to Iraq, the 26-year-old U.S. civilian had failed to find employment.