KABUL, Afghanistan - Pakistani authorities yesterday zeroed in on the alleged mastermind of a plot to send five Northern Virginia men to Afghanistan to kill U.S. troops, saying they hope the case could help unravel an extensive network of terrorist recruiters who scour the Internet for radicalized young men.

Investigators said they were hunting for a shadowy insurgent figure known as Saifullah, who invited the men to Pakistan after discovering them when one made comments approving of terrorost attacks on the Internet video site YouTube.

Saifullah guided the men once they were in Pakistan, attempting to help them reach the remote area in Pakistan's tribal belt that is home to al-Qaeda and its terrorist training camps.

But a Pakistani intelligence official who had been briefed on the case said yesterday that Saifullah was unsuccessful in convincing al-Qaeda commanders that the men were not part of a CIA plot to infiltrate the terrorist network. As a result, they were marooned for days in the eastern city of Sargodha, far from the forbidding mountains of the northwest that have become a terrorist haven.

"They were regarded as a sting operation. That's why they were rejected," said the official, who spoke on the condition of anonymity because of the sensitivity of the case.

The official said the men were undeterred and were still trying to acquire the right endorsements to gain access to the al-Qaeda camps when they were arrested by Pakistani law enforcement.

The case of the five - who remain in Pakistan and are being questioned by the FBI - underscores the critical role of recruiters in identifying potential terrorists and, perhaps more important, determining who can be trusted.

Since the 9/11 attacks, U.S. intelligence has made it a top priority to try to place human assets inside al-Qaeda. The organization's recruiters act as gatekeepers, keeping out those who are not serious about their commitment to holy war, and those who could be spies.

Would-be American recruits are treated by al-Qaeda with special scrutiny, analysts said. But they are also considered enormously appealing to the group because of their potential to access U.S. targets and because of their propaganda value.

But Evan Kohlmann, senior analyst with the U.S.-based NEFA Foundation, said that terror groups have also become much more cautious in recent years about who they allow in because U.S. intelligence agencies have become experts in their recruiting methods.

"If you're trying to sink someone into these groups, what better way than to follow the recruitment model that so many have already followed?" Kohlmann said.

The model is one that has become far more Web-based.

"Increasingly, recruiters are taking less prominent roles in mosques and community centers because places like that are under scrutiny. So what these guys are doing is turning to the Internet," Kohlmann said.

Recruiters who are satisfied that they have found a would-be terrorist who is serious, and not a spy, can then make the necessary introductions. "What they really serve as are facilitators, intermediaries to the jihadist world," Torres said.

In the case of the five men from Northern Virginia, their recruiter was unable to complete the introduction. The men - Ramy Zamzam, 22; Ahmad A. Minni, 20; Umar Chaudhry, 24; Waqar Khan, 22; and Aman Hassan Yemer, 18 - have not been charged with a crime. But investigators say they have proudly admitted to flying to Pakistan on Nov. 30 to join the war in Afghanistan.

The five were shifted from Sargodha yesterday to the provincial capital of Lahore, where they continued to face questioning. Pakistani officials said that while the men would ultimately be sent back to the United States to face charges, they were hoping to keep them in Pakistan while the investigation continues so they can use their statements to help track Saifullah and other members of his network.

The U.S. ambassador to Pakistan, Anne Patterson, met with Pakistani President Asif Ali Zardari yesterday to discuss the five, and the timing of their handover to the United States, Pakistani officials said.

The man known as Saifullah - Pakistani officials are unsure if it is his real name - was already wanted for his role in a spectacular attack earlier this year on the Sri Lankan cricket team as it visited Lahore for a tournament.

A Pakistani police official involved in the investigation said Saifullah is a member of the Pakistani Taliban, and that he first contacted the men in August.