But those who are reading Mr. Khan’s blog include officials at the Combating Terrorism Center who, since last year, have been training F.B.I. agents and analysts with the government’s joint terrorism task forces.

Center officials say Mr. Khan’s blog yielded confirmation of an important discovery: that a host of militant sheiks and scholars, dead and alive, are today far more influential than Osama bin Laden.

These men include Abdullah Yusuf Azzam, a mentor of Mr. bin Laden’s who promoted global jihad with his writings until his death in 1989, and the center’s findings helped the authorities conclude that Al Qaeda is but part of a larger, and diffuse, ideological movement.

Similar efforts to monitor online jihadists are under way in cities like Berlin, where intelligence and law enforcement officials have created a new multi-agency Internet center, and New York, where the police department this summer published a report on Islamic extremism that drew from the department’s online sleuthing.

Quite apart from the intelligence value of leaving the sites up, it’s not clear what can be gained by shutting them down. Operators of the sites have proved difficult to put behind bars, although legal experts offered mixed views on Mr. Khan’s blog.

“If I were a prosecutor I would look at treason,” said Andrew C. McCarthy, a former chief assistant United States attorney who led the 1995 terrorism prosecution against Sheik Omar Abdel Rahman. While treason has a large burden of proof and such a charge has been brought only once since World War II, Mr. McCarthy said the 1996 antiterrorism law should “make it much easier for prosecutors to win cases, especially in national security.”

“I’d also want to look at criminal solicitation,” Mr. McCarthy said. “I would probably scrub the information this guy is putting out, and see if it contains what you would need to convince a jury, specific commands to commit acts of violence.”