Intelligence officials conducted a technical analysis of the video released on an Islamic web site May 11 and determined "with high probability" that the person shown speaking on the tape — wearing a head scarf and a ski mask — is al-Zarqawi, a CIA official said, speaking on the condition of anonymity.
CBS News National Security Correspondent David Martin reports the CIA has not yet completed its analysis of the voice on the tape.
The person who is shown speaking in the video — determined to be al-Zarqawi — is then shown on the video decapitating American citizen Nicholas Berg, the official said.
Berg's body was found in Baghdad on Saturday. On Tuesday, an Islamic Web site released the video, titled "Sheikh Abu Musab al-Zarqawi slaughters an American infidel with his own hands."
The speaker on the video, now believed to be al-Zarqawi, reads a lengthy statement criticizing Islamic scholars and taunting the crusaders.
Standing alongside four other militants wearing headscarves and masks to disguise themselves, al-Zarqawi then kills Berg.
Al-Zarqawi is thought to be in Iraq, operating his own terrorist network, known simply as the "Zarqawi network." A specialist in poisons, he is thought to have extensive ties across the militant Islamic movement and is considered an ally of Osama bin Laden.
As recently as March, U.S. officials said al-Zarqawi's practice was not to make taped public pronouncements or take credit for attacks. However, in the last five weeks, he has increased his public profile with at least three recordings, including Berg's beheading.
Al-Zarqawi is believed to be behind well over a dozen high-profile attacks in Iraq, and many other acts of violence, which have killed hundreds.
Lt. Gen. Ricardo Sanchez, the top U.S. commander in Iraq, told reporters Thursday in Baghdad that it appears al-Zarqawi was responsible.
Asked about al-Zarqawi's whereabouts, Sanchez said, "We believe he's moving around the country."
The United States is offering a $10 million reward for information leading to his death or capture.
Martin reports that in at least one other case, a high-level al Qaeda operative has personally carried out a brutal killing. Khalid Shaikh Mohammed, the suspected Sept. 11 mastermind now in U.S. custody, is widely believed to have killed Wall Street Journal reporter Daniel Pearl.
Meanwhile Thursday, new details emerged about Berg's last weeks in Iraq — a timeline that has been contested by his family and the U.S. government.
Authorities in Baghdad denied that Berg, 26, was held in U.S. custody before he disappeared in early April, despite claims to the contrary by his family. The authorities said he had been held by Iraqi police for about two weeks and questioned by FBI agents three times.
In Baghdad, U.S. spokesmen Dan Senor said that "to my knowledge" Berg was not affiliated with any U.S. or coalition organization, nor was he ever in U.S. custody.
Iraqi police arrested Berg in Mosul on March 24 because local authorities believed he may have been involved in "suspicious activities," Senor said. He refused to elaborate, except to confirm that the Americans were aware Berg was in custody.
Berg was released April 6 and "was advised to leave the country," Senor added. Instead, Berg checked into a Baghdad hotel.
Berg had told friends he was arrested by Iraqi police in Mosul because he had an Israeli stamp in his passport. In e-mails released by his family, Berg wrote about his experiences in trying to track down and later meet an in-law in the Mosul area.
In Mosul, police chief Maj. Gen. Mohammed Khair al-Barhawi insisted Thursday that his department had never arrested Berg and maintained he had no knowledge of the case.
"The Iraqi police never arrested the slain American," al-Barhawi told reporters. "Take it from me ... that such reports are baseless."
Since Iraq remains under U.S. military occupation, it seems unlikely that the Iraqi police would have held Berg, or any other American, for such a length of time without at least the tacit approval of U.S. authorities.
"The Iraqi police do not tell the FBI what to do, the FBI tells the Iraqi police what to do," Berg's father, Michael Berg told the AP. "Who do they think they're kidding?"
The younger Berg told his family that U.S. officials took custody of him soon after his arrest and he was not allowed to make phone calls or contact a lawyer, his father said.
Kimmitt said U.S. forces kept tabs on Berg during his confinement to make sure he was being fed and properly treated because "he was an American citizen."
But the three FBI visits suggest American authorities were concerned about more than Berg's well-being. They may have had their own suspicions about what the young American was doing in Iraq.
Two e-mails Berg sent to his family and friends show he traveled widely and unguarded throughout Iraq, an unsafe practice rarely done by Westerners.
Shortly before Berg's disappearance, he was warned by the FBI that Iraq was too volatile a place for unprotected American civilians and that he could be harmed, a senior FBI official, speaking on condition of anonymity, said Wednesday.
Officials said the U.S. government warned Berg to leave Iraq, and offered him a flight out of the country, a month before his grisly death.
On April 10, four days after Berg was released from an Iraqi prison, an American diplomat offered to put him on a flight to Jordan, State Department spokeswoman Kelly Shannon said.
But Berg told the diplomat he "planned to travel overland to Kuwait and would call (his) family from there," Shannon said.
Michael Berg, said that although his son wanted to leave Iraq, he refused the flight offer because he thought the travel to the airport would be too dangerous.