Not limiting their activities to the earthly realm, U.S. and British spies have infiltrated the fantasy worlds of World of Warcraft and Second Life, conducting surveillance and scooping up data in the online games played by millions of people across the globe, according to newly disclosed classified documents.

Fearing that terrorist or criminal networks could use the games to communicate secretly, move money or plot attacks, the documents show, intelligence operatives have entered terrain populated by digital avatars that include elves, gnomes and supermodels.

The spies have created make-believe characters to snoop and to try to recruit informers, while also collecting data and contents of communications between players, according to the documents, disclosed by the former National Security Agency contractor Edward Snowden. Because militants often rely on features common to video games — fake identities, voice and text chats, a way to conduct financial transactions — U.S. and British intelligence agencies worried that they might be operating there, according to the papers.

Online games might seem innocuous, a top-secret 2008 NSA document warned, but they had the potential to be a "target-rich communication network" allowing intelligence suspects "a way to hide in plain sight." Virtual games "are an opportunity!" another 2008 NSA document declared.

But for all their enthusiasm — so many CIA, FBI and Pentagon spies were hunting around in Second Life, the document said, a "deconfliction" group was needed to avoid collisions — the intelligence agencies may have inflated the threat.

The documents do not cite any counterterrorism successes from the effort, and former U.S. intelligence officials, current and former gaming company employees and outside experts said in interviews they knew of little evidence that terrorist groups viewed the games as havens to communicate and plot operations.

Games "are built and operated by companies looking to make money, so the players' identity and activity is tracked," said Peter Singer of the Brookings Institution, an author of "Cybersecurity and Cyberwar: What Everyone Needs to Know." "For terror groups looking to keep their communications secret, there are far more effective and easier ways to do so than putting on a troll avatar."

The surveillance, which also included Microsoft's Xbox Live, could raise privacy concerns. It is not clear exactly how the agencies got access to gamers' data or communications, how many players may have been monitored or whether Americans' communications or activities were captured.

One U.S. company, the maker of World of Warcraft, said that neither the NSA nor its British counterpart, the Government Communications Headquarters, had gotten permission to gather intelligence in its game. Many players are Americans, who can be targeted for surveillance only with approval from the nation's secret intelligence court. The spy agencies, though, face far fewer restrictions on collecting certain data or communications overseas.

"We are unaware of any surveillance taking place," said a spokesman for Blizzard Entertainment, based in Irvine, Calif., which makes World of Warcraft. "If it was, it would have been done without our knowledge or permission."

A spokeswoman for Microsoft declined to comment. Philip Rosedale, the founder of Second Life and a former chief executive officer of Linden Lab, the game's maker, declined to comment on the spying revelations. Current Linden executives did not respond to requests for comment.