Khost, Afghanistan ? The night air was crisp, the horizon pitch black, as eight heavily armed and armored U.S. Special Forces soldiers and their local guides crept up to the small, one-story mud house, pausing about 750 feet away to find cover in ditches or behind rocks or trees.

With black masks pulled over their faces, the Americans trained their night-vision goggles on the house, looking for the targets, the guides recalled. Inside, the Americans had been told, might be Osama bin Laden’s top lieutenant and one of the leaders of the fallen Taliban regime. As satellites overhead monitored the area for movement, the Americans sent a pair of Pakistani contacts, wired with special communications equipment, to peek inside.

Each night for a week the U.S. team came back, sneaking over the border from Afghanistan into the treacherous and largely ungoverned tribal areas of Pakistan as soon as the sun went down to watch and wait until returning to the Afghan side of the border just before dawn, the guides recounted in interviews today. Ultimately, the Americans failed to find either Ayman Zawahiri, the second-ranking leader of al Qaeda, or Jalaluddin Haqqani, the senior Taliban commander from eastern Afghanistan.

But the guides’ account of the mission that ended two weeks ago opens a rare window into the secret war being waged on both sides of the border, a war fought through wits and rumor on a battlefield that shifts from one moment to the next. In this cat-and-mouse contest, America’s most elite commandos chase whispers of the enemy from one hostile village to another. So far, it seems they have been a step behind their elusive quarry.

“They’re always in movement,” said Mohammed Mustafa, one of the guides who helped the Americans look for the Taliban and al-Qaida figures in the tribal areas on the Pakistani side of the border last month. “We were very, very sure that they were there, but I don’t know what happened. As soon as we went there, we didn’t find anybody.”

At the moment, the U.S.-led coalition here has deployed multiple missions in the hunt for al-Qaida. There was a highly publicized search operation launched by British Royal Marines this week, but also smaller raids and reconnaissance conducted under the cloak of secrecy. A convoy of CH-47 Chinook helicopters ferried hundreds of Canadian soldiers at daybreak Saturday to mountain peaks in southeastern Afghanistan to begin yet another sweep of possible enemy hideouts in an action dubbed Operation Torii.

The most intriguing and delicate of the current missions, though, are taking place out of sight, across the Pakistani border, where intelligence reports suggest bin Laden and many of his surviving fighters may have fled.

Except for bin Laden, there is no one U.S. commanders would like to capture more than Zawahiri, founder of Egyptian Islamic Jihad, an extremist group that merged with bin Laden’s in 1998, who became the second-in-command of al-Qaida.