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 missing 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.
"In some cases we're using rubber boats to paddle down them; in other cases the guys are just walking," Kershaw said.
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.
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.
Last June, three soldiers from the 101st Airborne Division were killed in the same area. One died in an insurgent attack. The others were abducted and their mutilated bodies were found three days later.
Kershaw told reporters that after the June attack, insurgents paraded the two captives along the same road where the Saturday abduction occurred.
Two officers were relieved of their commands after the June killings. A military investigation concluded that the soldiers had been left alone for 36 hours in a poorly planned mission, an official familiar with the investigation told The Associated Press in Washington. 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. Mortar rounds also hit the U.S. Air Force base at Taji, 12 miles north of Baghdad, destroying one helicopter and damaging nine others, Iraqi police said. The U.S. military confirmed some aircraft were damaged in a mortar or rocket attack on the base but did not provide more details. In Baghdad, an explosion rocked the U.S.-controlled Green Zone one day after a mortar barrage that killed two Iraqis and wounded 10 other people there. The Green Zone houses the U.S. and British embassies as well as major Iraqi government headquarters and has been subjected to frequent attacks in recent weeks. Meanwhile, a prominent London think tank said Thursday that Iraq faces collapse unless the U.S. can bring Sunni insurgents and Shia extremists back into the political fold. "There is not ‘a' civil war in Iraq, but many civil wars and insurgencies involving a number of communities and organizations struggling for power," the Chatham House report said. "These current harsh realities need to be accepted if new strategies are to have any chance of preventing the failure and collapse of Iraq." 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 so-called sectarian death squads. Twenty-five of them were found on the Sunni-dominated western side of the Tigris River, indicating Shiite militias may be resuming operations after promising to lay low under the U.S.-Iraqi security crackdown.