Cyber vigilantes typically troll the Internet, searching message boards, Web sites and media sharing sites for incendiary postings from people with ties to terror groups like Al Qaeda. Using Arabic translation software, they monitor postings and even assume fake identities to join online conversations.

“It is very unlikely they will find something of significance in the Internet that the government doesn’t already know," Michael Radu, a senior fellow at the Foreign Policy Research Institute and an expert on terror-related Web sites, said.

A private intelligence contractor said winning the War on Terror isn't just about shutting down sites; it's about tackling the heart of the problem.

"For those working in the intelligence community, it becomes extremely costly, because then they have to go looking for the sites all over again," said [another] private intelligence contractor, noting that U.S. intelligence often knows of the sites for a long time and monitors their traffic to look for clues to their origins, creators and visitors.

When the site comes down, he said, intelligence investigations can be ruined.