Terrorist Abu Musab al-Zarqawi was the masked man who beheaded an American civilian in Iraq, U.S. intelligence officials concluded Thursday, leaving other questions unresolved about Nicholas Berg's final days and his contacts with U.S. and Iraqi authorities.

In an odd twist, it also emerged Thursday that the FBI questioned Berg in 2002 about an e-mail address traced to him that was used by an acquintance of terrorism suspect Zacarias Moussaoui (search). Investigators concluded that Berg had nothing to do with Moussaoui.

Through a technical analysis, intelligence officials were able to determine "with high probability" that the speaker on a video showing Berg's beheading was al-Zarqawi, said a CIA official, speaking on condition of anonymity. The same person is shown decapitating Berg, the official said.

Three days after Berg's body was found on Saturday, an Islamic Web site released a video, titled "Sheikh Abu Musab al-Zarqawi (search) slaughters an American infidel with his own hands."

U.S. authorities consider al-Zarqawi an ally of Usama bin Laden and say he is running his own terrorist operation.

The video shows five armed men disguised by head scarves and masks line up behind Berg, who is seated on the ground. Standing in the middle, the man identified as al-Zarqawi reads a lengthy statement that criticizes Islamic scholars and the "shameful photos" of the humiliation of men and women at the Abu Ghraib prison. He then decapitates Berg.

It is unclear when and how Berg, a self-employed telecommunications businessman, was captured. Accounts of his detention in Mosul in late March are also conflicting.

U.S. officials insist Berg was arrested by Iraqi police for involvement in "suspicious activities." The Mosul police chief has denied that. An April 1 e-mail from a U.S. consular official in Iraq, provided by Berg's family, said he was being detained by the U.S. military.

But State Department spokeswoman Kelly Shannon said late Thursday that the diplomat had been given erroneous information from the U.S.-led Coalition Provisional Authority (search) in Iraq. Not until one day after Berg's release from jail was the diplomat told that Iraqi police had held Berg, Shannon said.

"As Mr. Berg had been released, the consular officer did not convey this information to the family because he was released, thankfully — he was released," Shannon said late Thursday. "And we thought he was on his way."

The FBI visited Berg three times before his April 6 release, a U.S. spokesman in Iraq, Dan Senor, said Wednesday. The agents told Berg that Iraq was too dangerous for unprotected American civilians.

Berg wrote his parents after his release that federal agents had questioned him about whether he had ever built a pipe bomb or had been in Iran.

Berg's family members have called on the government to tell all it knows about its contacts with the 26-year-old. They have blamed U.S. authorities for detaining him in Iraq until the anti-American violence worsened.

Berg had caught the FBI's attention before.

Speaking to reporters outside his home in West Chester, Pa., Berg's father, Michael, said Thursday that his son was investigated by the FBI over contact he had with a terrorism suspect while he was a student at the University of Oklahoma.

A senior law enforcement official, speaking on condition of anonymity, said Berg volunteered information about the 2002 investigation when he was detained in Iraq. The official said that an e-mail address traced to Berg had been used by an unidentified individual with purported connections to terrorism.

The investigation showed that Berg had never met the suspect individual and had not given the e-mail address to that person. Investigators concluded that Berg's e-mail address had been spread among dozens of people with links to the university.

The suspect individual appears to have been acquainted with Moussaoui, an Al Qaeda adherent now in federal custody and awaiting trial on conspiracy charges stemming from the Sept. 11 attacks, the official said.

Moussaoui attended flight school in Norman, Okla., home of the university.

In what may be Berg's last contact before his kidnapping, he checked out of Baghdad's Fanar Hotel on April 10 around 7 a.m., according to the hotel receptionist. American businessman Andrew Robert Duke and the staff there said Berg left some of his belongings in storage, with plans to return.

"Inshallah [God willing], I will be back in a few days," the receptionist said Berg told her.

A porter watched him walk down the street with his bags.

Berg's killing was reminiscent of the 2002 death of Wall Street Journal reporter Daniel Pearl (search), who was decapitated while a video camera captured the slaying.

Pearl's killing is alleged to have been committed by Khalid Shaikh Mohammed (search), a senior Al Qaeda operative and accused Sept. 11 mastermind.

The FBI is leading the investigation into Berg's death. Intelligence agencies are analyzing the videotape, which is of relatively low quality.

Al-Zarqawi appears to be seeking an increasingly high-profile presence. As late as March, U.S. officials said he was not known for making public statements or taking credit for attacks. But in the past five weeks, he has released three recordings, including the beheading.

The military has increased the reward for his killing or capture to $10 million in January.

Although al-Zarqawi has terrorist ties stretching from Europe to Central Asia, he is believed to have been working out of Iraq for some time.

The Jordanian-born Palestinian is a poisons expert. He is thought to be responsible for hundreds of deaths in Iraq. Last month, al-Zarqawi was sentenced to death in absentia in Jordan for masterminding the successful 2002 plot in the murder of Laurence Foley, a diplomat and administrator of U.S. aid programs in Jordan.