In early 2006, Klein revealed details about AT&T secretive cooperation with the National Security Agency's
mass surveillance program. He found that the NSA was using a
splitter to intercept and monitor all internet and other data traffic, which was then directed to a secure room at AT&T's facility, called
Room 641A. He came across blueprints and documents outlining the technology, the program's existence, and AT&T's involvement with the NSA. He also found evidence that the program was active in other cities across the United States. This surveillance initiative followed the Patriot Act, which granted American defense and intelligence agencies broader powers after the
9/11 attacks. The
Electronic Frontier Foundation (EFF) filed a lawsuit shortly afterwards. In 2009, Klein wrote a book titled
Wiring Up The Big Brother Machine...And Fighting It, in which he wrote about his experience discovering the relationship and his efforts to tell the public. ==Personal life and death==