An official said that military commanders assume at least two of the missing soldiers are alive and being held by an al Qaeda-affiliated group.

Authorities detained 14 people Monday within the search area, said US military officials.

The three US soldiers have been missing since May 12 when attackers raided a US military observation post just outside Mahmoudiya, south of Baghdad, in an area known as the Triangle of Death. Four American soldiers and an Iraqi soldier were killed in the attack.

Iraqi forces have rounded up dozens of individuals as part of the search. At least 15 are being processed into US detention facilities, and more than 250 are being processed into Iraqi detention facilities, according to Col. Mike Kershaw, commander of the 2nd Brigade Combat Team, 10th Mountain Division.

Only those who may have directly participated in the May 12 attack, have first-hand knowledge of the attack, or can help locate the missing soldiers, are being placed under US military custody, Kershaw said.

Of those, at least two are believed to be directly responsible for the attack, a senior US military official said.

During Monday's search, troops recovered a piece of physical evidence that indicates one of the detainees held in the wake of the ambush may be directly involved, Kershaw said.

Four people were detained and released. Sources had told the US military that relatives of individuals associated with the May 12 attack lived in the Jurf al-Sakhr area.