The commander of US forces in Iraq, Gen David Petraeus, has said he knows who captured three US soldiers, who have been missing since last Saturday.
A massive search is underway for the three missing US troops
In an interview with the newspaper Army Times, he said the kidnappers were linked to al-Qaeda.
In the interview, given on Friday, Gen Petraeus also said he thought at least two of the soldiers were still alive.
Five soldiers were killed and three kidnapped last Saturday in an attack near Mahmudiya, south of Baghdad.
Gen Petraeus said a tipster had given the US military information on who carried out the attack and took the missing soldiers.
"We know who that guy is," Gen Petraeus told Army Times. "He's sort of an affiliate of al-Qaeda.
"He's the big player down in that area. We've tangled with him before.
"Somebody's given us the names of all the guys that participated in it and told us how they did it, and all the rest of that stuff.
"Now we have to verify that at some point in time, but it sounds spot on."
The US military announced earlier that it had detained nine people in Amiriya, a stronghold of Sunni insurgents 40km (25 miles) west of Baghdad, who are suspected of involvement in the attack .
Spc Alex Jimenez, Pte Joseph Anzack and Pte Byron Fouty, all from the 10th Mountain Division, have been missing since the attack on their two vehicle convoy early on 12 May.
Four other US troops and one Iraqi soldier with them were killed.
The US military has offered rewards of up to $200,000 (£100,000) for information leading to the return of the three.
Thousands of US and Iraqi soldiers have been scouring the farms and villages near Mahmudiya, 30km (20 miles) south of Baghdad, looking for the soldiers.
Gen Petraeus also said he thought that two of the soldiers might be alive but he was not sure.
"At one point in time there was a sense that one of them might have died, but again we just don't know."
In Washington, the Pentagon said that reports on Arabic-language websites that two US soldiers had been found decapitated in Iraq were "false" and "can't be substantiated".
In other events in Iraq:
- At least one US soldier was killed and another wounded on Saturday in an attack on troops searching for the missing soldiers
- Twelve other US soldiers have died since Thursday in several different incidents
- Fifteen Kurds were killed in Hamid Shifi, 100km (60 miles) north-east of Baghdad, by men wearing Iraqi army uniforms, the Associated Press news agency reported
- The bodies of 20 Iraqi men were found in different locations in Baghdad.