The Federal Bureau of Investigation is monitoring the Web for potential terrorist activity, the Internet giant Google says in a transparency report.
FBI used National Security Letters - a form of surveillance that privacy watchdogs call "frightening and invasive" - to surreptitiously seek information on Google users, the web giant revealed.
But it can't say how extensive the surveillance is.
"The FBI made requests for data on as many as 2,000 Google accounts last year," it said on Tuesday in the report.
The report, 'Transparency Report: Shedding more light on National Security Letters', was posted by Richard Salgado, the legal director of law enforcement and information security at Google.
"The FBI can seek the 'name, address, length of service, and local and long distance toll billing records' of a subscriber to a wire or electronic communications service" by filing a National Security Letter (NSL), the report said.
Google has received as many as 4,000 NSLs since 2009, requesting information for as many as 10,000 accounts.