When U.S. troops, rifles drawn, burst into a headquarters of Iraq's much-feared intelligence service in Baghdad last week, their mission was to seize thousands of files documenting the nation's foreign spy network, its secret weapons purchases, its executions of civilians and the location of chemical and biological weapons.

Instead, they found nothing.

The safes, shelves and locked rooms of the Mukharabat headquarters were empty, meticulously and "professionally" cleaned out in what U.S. intelligence officials now say they suspect was part of an escape planned by many of Iraq's top security, military and political leaders.

As U.S. troops Friday began creeping around the dark, seemingly endless tunnels beneath Baghdad as part of an intensified effort to find deposed Iraqi president Saddam Hussein and his senior officials, the mystery of their whereabouts only deepened. To aid in the manhunt, the U.S. military gave soldiers in Iraq decks of playing cards bearing the photographs of Hussein and 54 other of the most sought-after members of his regime.

The week's best guess -- that Hussein's inner circle may have fled to his hometown of Tikrit to prepare for a bloody last stand -- seemed to evaporate Friday as images from U.S. Air Force unmanned drones showed Hussein loyalists abandoning their posts there. U.S. warplanes have bombarded the town since the war began, significantly weakening combat units and destroying underground bunkers, said defense and intelligence officials.

By the end of the day Friday, Syria was left as the most likely escape route for Hussein and his top advisers. U.S. military commanders stepped up reconnaissance and unmanned surveillance flights along the 375-mile border between Iraq and Syria, a traditional smuggling route, and moved more troops to the area.

President Bush warned Syria against providing sanctuary for the Iraqi leadership. U.S. intelligence officials said that some leaders' family members had headed west to Syria before the war began and that others had continued to try to escape there.

Syrian spokesman Imad Moustapha said the U.S. allegations against his country were "a fabrication." Syria had closed its borders with Iraq shortly before the war began, he said. "This is an unfair campaign," Moustapha said. "We don't even have an ambassador to Iraq. How would we know what's going on there?"

A senior U.S. military official, on the other hand, said that one of Iraq's top nuclear scientists had taken refuge in Syria.

On the question of whether Hussein was alive, one senior administration official said the informal "dead or alive" needle had "inched more in the dead direction" as communication intercepts picked up discussion about Hussein's death from people "who ought to be in the position to know."

"They were telling each other they think he's dead," the official said. But "we don't know if they really know or not, or if they are trying to fool us."

CIA and military intelligence analysts have been wading through a mass of documents, intercepts, imagery and reports from Iraqis in an effort to determine where Hussein and almost his entire retinue of top civilian and military leaders have gone.

"There's a wide array of sources that provide us information," Brig. Gen. Vincent K. Brooks told reporters Friday at U.S. Central Command headquarters in Qatar. He said coalition forces were trying to prevent Iraqi officials from leaving "by air, by smuggling [and] by movement in vehicles," but "it is very difficult to be able to cast the net over all of Iraq and prevent any movement at all."

"We can't be everywhere in the country, and there will be no shoulder-to-shoulder, arm-in-arm type of fence that goes around all of Iraq," he said.

A lack of good leads has forced the U.S. military to "look for places where there's likely movement," Brooks said. "We take information from others as to where there might be movement, who might be moving, and we try to take appropriate action to prevent that from occurring."

That approach led to heavy bombing on Tikrit and to the strong coalition push to capture Qaim, the town near the Syrian border from which Iraq launched missiles at Israel during the 1991 Persian Gulf war.

Fighting in Qaim was heated Thursday, but Friday U.S. military officials received reports that the Iraqis were preparing to surrender. Early Friday, Brooks described Qaim as "located on a very critical crossroad between Iraq and Syria. And given some of the reports of infiltration attempts or exfiltration attempts by regime leaders or by foreign fighters, that remains a concern to us."

An Iraqi colonel who supervised the border crossing between Iraq and Syria had been captured and turned over to U.S. interrogators, who would be seeking details about the routes Iraqi leaders or their families might have taken to Damascus.

The Army's top intelligence officer, Lt. Gen. Robert W. Noonan Jr., said Friday on NBC's "Today" program that the search of tunnels under Baghdad was proceeding carefully because of concern about booby traps. In addition, he said, the tunnels -- built, "we strongly believe, to hide things" -- may also house chemical or biological agents or weapons.

Veteran Russian diplomat and intelligence officer Yevgeny Primakov added his name to the list of those who believe Hussein would die before fleeing his country to save himself. Three days before the United States dropped its first bombs in an attempt to kill the Iraqi leader, Primakov met with Hussein in his palace on a secret mission to persuade him to leave.

He told reporters in Moscow that Hussein had listened to him "without a word." Then he made a brief reference to Russia trying to talk to him before the first Gulf War, then "patted me on the shoulder and walked out."

Primakov said that on an earlier mission to Baghdad, in February, Hussein had told him, "I was born in Iraq, and I will die in Iraq."

Staff writer Bradley Graham and researcher Margot Williams contributed to this report.

At U.S. Central Command, Brig. Gen. Vincent K. Brooks shows deck of cards featuring mug shots of Iraqi leaders wanted by the coalition.