BAGHDAD - U.S. officials expressed cautious optimism yesterday that three missing American soldiers were still alive even as troops drained canals and questioned children in the search for the troops feared captured by al-Qaeda in Iraq.

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. troops 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," Martin said. "But we have an obligation to follow on every intelligence tip."

He added: "There is cautious optimism that in fact these soldiers can be found alive. 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 declined to say more.

Martin said some of the Iraqis detained in the search might 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, of whom 150 had provided intelligence, Kershaw said.

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 near the two vehicles indicated the soldiers - from an infantry regiment with the 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.

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

"In some cases we're using rubber boats to paddle down them," Kershaw said. "In other cases, the guys are just walking."

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. Kershaw said two young brothers provided the first lead about which direction the insurgents had followed after capturing the troops.

An al-Qaeda 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.

Describing Saturday's attack, Maj. Gen. Rick Lynch, the commander of the Third Infantry Division, said the seven U.S. soldiers and an Iraqi army interpreter, in two humvees, were watching over bomb craters where insurgents had previously planted roadside bombs. The objective was to prevent bombs from being placed again in that spot. The soldiers were 500 yards south of a patrol base, Lynch said.

That's why "the determination was made that two humvees were indeed adequate," to watch over the craters, Lynch said. The soldiers, he added, had also taken measures to protect their positions, including running concertina wire on either side of the humvees.

He said the soldiers were attacked from multiple directions. The attackers managed to breach the concertina with wire cutters or some other means. They also carried hand grenades and other explosives, which may have been used in setting fire to the humvees, Lynch said.

The attackers had vehicles nearby, he added, because there were signs on the ground that "bodies were moved or dragged and placed in vehicles. The drag marks stopped at the tire tracks of vehicles."

The area is inhabited by Sunni Arab clans that had been loyal to Saddam Hussein, and al-Qaeda has been active there for several years.

On Other Fronts: 3 Troops Die

Three American soldiers were killed and one was wounded in a roadside bombing

south of Baghdad, the U.S. military reported yesterday. The statement provided no other details, and it was unclear if the victims were part of the search for three missing soldiers.

Mortar rounds hit the U.S. air base at Taji, 12 miles north of Baghdad, destroying one helicopter and damaging nine, Iraqi police said.

Across the country, at least

58 Iraqis were killed or

found dead in bombings, shootings and mortar attacks, police said. They included 42 bullet-riddled bodies of apparent victims

of sectarian death squads.

- Associated Press

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This article contains information from the Washington Post.