Baghdad ? U.S. officials expressed cautious optimism Thursday that three missing American soldiers are still alive even as troops drained canals and questioned children in the search for the troopers feared captured by al-Qaida.

Blackened asphalt and spare parts are seen Thursday at the site of an attack that left four U.S. soldiers and an Iraqi translator dead, and three other soldiers missing in Cargouli village, near Youssifiyah, 12 miles south of Baghdad.

FBI agents and Australian forensic experts have joined the search for the soldiers, who disappeared after an attack south of Baghdad on Saturday in which four U.S. troopers and one Iraqi were killed.

Lt. Col. Randy Martin, a U.S. military spokesman, said five days of searches had produced a number of leads that “point to the fact that these men are still alive.”

“There are also reports to the contrary. But we have an obligation to follow on every intelligence tip,” Martin said.

“There is cautious optimism that in fact these soldiers can be found alive,” Martin said. “That’s what we pray for, that’s what we hope for.”

Col. Michael Kershaw, a brigade commander, said some physical evidence associated with the missing soldiers had been found, raising hopes they were alive. He refused to say more.

Martin said some of the Iraqis detained in the search may have been directly involved in the ambush, which occurred along a palm-lined rural road near Youssifiyah, 12 miles south of Baghdad. More than 500 people have been questioned so far, of whom 150 had provided intelligence, according to Kershaw.

Four days after the attack, the ambush site remained littered with debris and pieces of armor on a swath of blackened asphalt on a palm-tree lined road, guarded by Humvees.

Shell casings found around the two vehicles indicated the soldiers – from D Company, 4th Battalion, 31st Infantry Regiment, 10th Mountain Division – had put up a fight but were overwhelmed by automatic weapons fire and rocket-propelled grenades, military officials said.

Three unexploded roadside bombs were found near their burned-out vehicles, they said.

An al-Qaida front organization – the Islamic State of Iraq – claimed responsibility for the attack and warned the Americans to call off the search or risk the safety of the captives.

In efforts to find the soldiers, Kershaw said troops had drained at least two of the canals that crisscross the Euphrates River flood plain. Armed patrols have walked for hours along the banks of others looking for any sign of the soldiers.

The Australian forensics experts and two FBI agents have been brought in for specialized investigative work, including questioning women and children who had been separated from the military-age men.

The soldiers were attacked while manning an observation post composed of two Humvees surrounded by concertina wire that had been breached, he said. They were watching for insurgents placing roadside bombs about 800 yards from their patrol base in a rural villa.

The area is inhabited by Sunni Arab clans that had been loyal to Saddam Hussein, and al-Qaida has been active there for several years. The area has been nicknamed the “triangle of death” because of frequent attacks against Shiites traveling between Baghdad and shrine cities to the south.

While the search was under way Thursday, three American soldiers were killed and another was wounded in a roadside bombing south of Baghdad, the U.S. military reported. The statement provided no more details, and it was unclear if the victims were part of the search.