BAGHDAD — Attackers who ambushed a U.S. military convoy apparently confiscated the dog tags of one of the four American soldiers killed, the military said today, as it pressed its massive search for three U.S. soldiers feared captured by al-Qaida five days ago.

Maj. Webster Wright, a spokesman for the Second Brigade of the 10th Mountain Division — the unit that was attacked and that is leading the search — said Saturday's assault in an insurgent stronghold south of Baghdad apparently used rocket-propelled grenades and small arms fire to severely damage the two Humvees in the stationary U.S. convoy.

He said the dog tags of one of the four American soldiers who died were missing and apparently had been taken from the scene by the attackers. That could explain why the military has only been able to identify three of the four dead U.S. soldiers.

About 4,000 U.S. troops and 2,000 Iraqis are searching for the three U.S. soldiers feared captured by al-Qaida during the ambush, which also killed one Iraqi soldier.

Maj. Gen. Rick Lynch, commander of U.S. troops south of Baghdad, said Wednesday that the U.S. was offering rewards of up to $200,000 for information on the missing soldiers' whereabouts.

"We've done so much as to drain canals after a report that the bodies were in a canal," Lynch said. "So we're leaving no stone unturned."

Lynch said he was optimistic that the three soldiers would be found alive, and the search remained focused on the area where they went missing in a rural area near the town of Youssifiyah, 12 miles south of Baghdad.

Wright said Cargouli, a Sunni tribe, dominates the area where many former intelligence agents, Baathists and Republican Guards from Saddam Hussein's government live.

An al-Qaida front group, the Islamic State of Iraq, has said it captured the U.S. soldiers and warned the Americans in a Web statement on Monday to call off the hunt "if you want their safety."

Meanwhile, attacks by suspected insurgents continued in other parts of Iraq.

For the second time in a week, they set off a bomb near a bridge in southeastern Baghdad today, killing two civilians and wounding five, police said.