Al-Qaeda suspect Abu Musab al-Zarqawi probably carried out the beheading of US contractor Nicholas Berg in Iraq, CIA officials say.
Zarqawi is said to be behind many attacks against coalition troops
US intelligence agencies have been analysing the video of the execution shown on a Malaysian-based website.
It was highly probable that it was Zarqawi who wielded the knife, an unnamed CIA official said.
The graphic images of the 26-year-old's death prompted shock and outrage all over the world.
His killers said they were avenging the abuse of Iraqi detainees by US forces in Iraq.
They had offered to exchange him for inmates at Baghdad's Abu Ghraib prison but the coalition authorities had, they said, refused.
The grainy pictures of the murder were preceded by a caption announcing that the Jordanian-born Zarqawi would carry out the killing.
"After the intelligence community conducted a technical analysis of the... video, the CIA assesses with high probability that the speaker on the tape is Abu Musab al-Zarqawi, and that person is shown decapitating American citizen Nicholas Berg," a CIA official said.
Berg's execution has caused horror across the world
In February, US officials in Iraq said they had uncovered what they believed to be a plot by Zarqawi to foment sectarian violence in the country.
They believe he was behind a number of high-profile attacks and have offered a $10m reward for information leading to his death or capture.
FBI visited Berg
Mr Berg - a telecoms worker - was arrested two months ago in Iraq.
According to his family, he was held for 13 days before being released without charge. It is not clear who arrested and held him.
Agents for the American FBI say they interviewed him when he was in the custody of Iraqi police in the northern city of Mosul, but other reports say he was not held by the Iraqis.
Mr Berg's father, Michael, said his son had been trying to come home before the violence in Iraq worsened.
The family have partly blamed the US-led coalition for creating the circumstances in which he was abducted, about a month ago.
They were holding a memorial service on Friday.