US and British spies have reportedly infiltrated online games such as World of Warcraft in an effort to identify terrorist threats, according to media reports.
The undercover agents reportedly operated in virtual universes to observe messaging and payment systems.
The NSA allegedly warned that such online games could allow intelligence targets to hide in plain sight.
Virtual universe games draw millions of players from around the globe.
News of the operation was broken by the New York Times, the Guardian and ProPublica on Monday using leaked confidential government information obtained by whistleblower Edward Snowden.
The media reports allege US and UK spies spent years investigating online games including Second Life for potential terrorist activity.
One leaked document published by the New York Times claims such video games could be used for recruitment or to conduct virtual weapons training.'Without our knowledge'
The NSA is said to have extracted World of Warcraft account data and attempted to link it to Islamic extremism and arms deals, according to the Guardian.
The popular online fantasy game, which at one point boasted upwards of 12 million subscribers, has reportedly attracted users such as embassy employees, scientists and military and intelligence officials.
At one point during the investigation, so many national security agents were reportedly playing video games that a "deconfliction" group was created to ensure they were not inadvertently spying on one another.
However, the documents obtained by former NSA contractor Mr Snowden and cited by the media did not specify if any terrorist plots had been foiled by the effort.
A spokesman for World of Warcraft's parent company Blizzard Entertainment told the Guardian they were not aware any surveillance had been conducted.
"If it was, it would have been done without our knowledge or permission," the spokesman said.