US Army Pfc. Kristian Menchaca is shown in this undated family photograph. Menchaca is one of the two US soldiers found dead after they became missing on Friday at a checkpoint in Yusufiya, a town in an area south of Baghdad. [Reuters]

Caldwell said only a single vehicle carrying the three US soldiers was attacked. A witness has said two other Humvees were in the area and went after the assailants, while seven masked gunmen ambushed the third Humvee.

Some 8,000 Iraqi and US troops searched for the missing soldiers. One US soldier died and 12 were wounded during the search, Caldwell said, adding that coalition troops killed two insurgents and detained 78. The troops received 66 tips, 18 of which were considered worthy of follow up.

The Mujahedeen Shura Council, an umbrella organization of five insurgent groups led by al-Qaida in Iraq, posted an Internet statement Monday claiming it was holding the American soldiers captive and that "we shall give you more details about the incident in the next few days, God willing."

On Tuesday, after Iraqi officials disclosed that the bodies were found, the Shura Council posted another Web statement, saying al-Zarqawi's successor had "slaughtered" the soldiers. The language in the statement, which could not be authenticated, suggested the group was saying the men were beheaded.

"With God Almighty's blessing, Abu Hamza al-Muhajer carried out the verdict of the Islamic court" calling for the soldiers' slaying, the statement said.

The US military has identified al-Muhajer as an Egyptian associate of al-Zarqawi also known as Abu Ayyub al-Masri. If confirmed, the killings would be the first acts of violence attributed to al-Muhajer since he was named the new leader of al-Qaida in Iraq in a June 12 Web message by the group.

Al-Zarqawi made al-Qaida in Iraq notorious for beheadings and was believed to have killed two American captives himself, Nicholas Berg in April 2004 and Eugene Armstrong in September 2004. A dozen Americans are still missing in Iraq, Caldwell said.

Just hours before Tucker and Menchaca disappeared Friday, a US airstrike killed a key al-Qaida in Iraq leader described as the group's "religious emir," Caldwell said.

Mansour Suleiman Mansour Khalifi al-Mashhadani, or Sheik Mansour, died along with two foreign fighters in the same area where the soldiers' bodies were found. The three were trying to flee in a vehicle.

Al-Mashhadani, identified as an Iraqi in his late 30s, was "a key leader of al-Qaida in Iraq, with excellent religious, military and leadership credentials" and tied to the senior leadership, including al-Zarqawi and his alleged replacement, Caldwell said.

US forces captured Mansour in July 2004 because of his ties to the militant groups Ansar al-Islam and Ansar al-Sunna, but the military let him go because he was not deemed an important terror figure at the time.

Tuesday's violence across Iraq included at least three bombs striking Baghdad despite a security crackdown launched nearly a week ago.

In the bombing of the home for the elderly, an 18-year-old Sunni wearing an explosives belt blew himself up as senior citizens were lined up to collect monthly pensions. Two elderly women were killed and three people were wounded.

Police said the motive was unclear, but sectarian tensions have been worsening in the predominantly Shiite city of Basra.