U.S. soldiers raided suspected safehouses near the Euphrates River south of Baghdad on Monday in their search for three captured comrades but found them empty after the militants apparently were tipped off and fled, a military spokesman said.

It was the latest in a series of frustrations for exhausted U.S. troops hunting for any sign of the trio missing since a May 12 assault on an outpost by insurgents linked to Al Qaeda. Four other Americans and an Iraqi soldier died in the attack.

Maj. Webster Wright, a spokesman for the 10th Mountain Division's 2nd Brigade Combat Team, said 27 airborne operations had been staged during the widespread search the last nine days, dropping off soldiers to gain the element of surprise and avoid bomb-studded roads.

"It's very quick, it's very sudden," he said. "You can come from any different angle and drop down on a house without being pinned to the roads."

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But he said the insurgents were catching on to U.S. tactics and had fled ahead of raids near the river Monday.

"We went after a couple of objectives and they had some warning systems out," Wright said. "They were able to slip away from us."

He said planners were looking at targets to the west of the division's area of operations. He wasn't more specific, but that would likely include volatile Anbar province, a vast desert area that stretches to the borders of

Syria, Jordan and Saudi Arabia. Anbar is considered a key smuggling route for insurgent groups including factions linked to Al Qaeda.

"We are looking at all of the possibilities that the enemy might be taking. Some of those might options might be to the west," Wright said.

Maj. Gen. Rick Lynch, commander of U.S. troops south of Baghdad, said in a CNN interview that 4,000 American soldiers and 2,000 Iraqis were involved in the search.

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He expressed "cautious optimism" that the three missing men — Pfc. Joseph J. Anzack Jr., 20, of Torrance, Calif.; Spc. Alex R. Jimenez, 25, of Lawrence, Mass.; and Pvt. Byron W. Fouty, 19, of Waterford, Mich. — were still alive.

"We won't stop until we find our fallen comrades," he said, adding that the military was following up 249 intelligence reports, the majority of which said the three soldiers were still alive.

"We're pursuing all leads with a passion, but right now we believe our soldiers are still alive," Lynch said. "Each day that passes when we don't see proof of life, it causes us concern."

Lynch said more than 1,000 people had been detained for questioning, and two had confessed to taking part in the May 12 attack. "So they're giving us actual intelligence and we continue to follow through," he said.

Meanwhile, the platoon whose soldiers were attacked at an observation post near the village of Qarghouli began to regroup, receiving eight replacements for those killed and captured.

The platoon leader, Lt. Morgan J. Spring-Glace, 25, of Worcester, Mass., said he took the new men out on their first operation Monday to search for those behind the attack.

"We've had two missions in the past 12 hours, and this last one my new guys went on, and we just got them like 12 hours ago. I don't even know all of their names. It's been a hectic schedule for everyone," he said.

His soldiers from Company D, 4th Battalion, 31st Infantry Regiment — nicknamed the "Polar Bears" — fought exhaustion and dehydration as they searched farm fields and houses in temperatures above 100 degrees. Even the search dogs were haggard from the heat.

At one house, soldiers were told to check everything, looking for T-shirts and pieces of uniforms, anything that the missing soldiers might have been wearing that night.

"They've been keeping busy. These guys are walking themselves to death. These guys are almost in some cases getting run into the ground," Spring-Glace said.