Story Highlights• U.S. troops target area near site where missing soldiers' post ambushed May 12
• Fourteen detained Monday; U.S. cites evidence of one detainee's involvement
• May 12 ambush south of Baghdad also killed four U.S. troops, Iraqi soldier
• U.S. says it killed ringleader of January attack that left five Americans dead
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BAGHDAD, Iraq (CNN) -- U.S. troops on Monday focused their intense search for three missing comrades to a nine-mile (15-kilometer) radius around the site of the May 12 attack from where the soldiers were taken.
An official told CNN 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, U.S. military officials told CNN's Arwa Damon, who is embedded with American forces involved in the search. (Watch how U.S. forces are chasing leads in search of the soldiers' whereabouts )
The three U.S. soldiers have been missing since May 12 when attackers raided a U.S. 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. (Interactive: Details about ambushed soldiers)
Iraqi forces have rounded up dozens of individuals as part of the search. At least 15 are being processed into U.S. 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.
Some of the detainees may be released after further questioning.
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 U.S. military custody, Kershaw said.
Of those, at least two are believed to be directly responsible for the attack, a senior U.S. military official said.
"We continue with their interrogation to gather whatever information we can," the military official told CNN's Barbara Starr. "We have historical information that ties these two detainees to known Sunni extremist groups in our area of operation."
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.
Acting on information from detainees and other sources, two Striker companies Monday also crossed the Euphrates River into the town of Jurf al-Sakhr, an area that has not previously been searched.
Four people were detained and released. Sources had told the U.S. military that relatives of individuals associated with the May 12 attack lived in the Jurf al-Sakhr area.
On Sunday, U.S. forces drained a canal that leads from the Euphrates to the village of Janabi but turned up nothing of interest. Local Shiite villagers had said they spotted body parts floating in the water, but the mayor and a police official dismissed that claim.
U.S. military spokesman Maj. Gen. William Caldwell expressed optimism that the three will be found.
"We're not going to give up on our soldiers," Caldwell told CNN's "Late Edition With Wolf Blitzer" on Sunday. "We have every reason to believe at this point still that they probably are alive."
In an interview with the Army Times on Friday, Gen. David Petraeus, commander of U.S. forces in Iraq, said he believed at least two of the soldiers are still alive. (Read more of what Petraeus said about the search for the missing soldiers)
A senior U.S. military official at the Pentagon told Starr that the military is receiving reports saying that some of the soldiers are alive, and an equal number of reports saying they are all dead.
"I choose to remain positive, but with each passing day with no evidence of life, the likelihood of soldiers being found alive is less," said the official, who spoke on condition of anonymity because of the sensitivity of the intelligence. "We have not ruled out anything, and we continue to pursue every lead with all assets available."
U.S.: Karbala attack leader killed
U.S. forces have killed a man identified as the ringleader of the January 20 attack on U.S. troops in Karbala that left five Americans dead, Caldwell, the military spokesman, told CNN's "Late Edition." (Watch what happened during the bloody attack on U.S. forces at Karbala )
Sheikh Azhar al-Dulaymi was killed Saturday during an operation in Baghdad's Sadr City neighborhood, according to an unclassified U.S. military presentation obtained by CNN. But Caldwell said the death occurred Friday.
"We've been pursuing this guy relentlessly," he said. "Anybody who kidnaps an American soldier and murders them we're going to continue to hunt down."
In the January attack, a group of armed insurgents "disguised as American soldiers, driving American vehicles, speaking English" raided a military compound in Karbala, killing one U.S. soldier at the scene and kidnapping four others, who were later slain, Caldwell said.
According to the military's release, al-Dulaymi -- who was armed -- was trying to hide on the roof of a building when U.S.-led coalition forces arrived on the scene to capture him.
The release did not detail how he was wounded but said he died as coalition forces were taking him to a military medical facility.