Mark Klein, a former AT&T technician turned whistleblower who exposed mass surveillance by the U.S. government, has died at age 79.

Klein went public in 2006 with documents revealing that the NSA was using a secret room in an AT&T hub in San Francisco to tap into the backbone of the internet.

Behind the door of the now-infamous Room 641A, optical splitting wiretaps were creating an identical copy of raw internet traffic and funneling it back to the NSA. 

Klein’s disclosure was confirmation that the U.S. government was accessing the internet data on millions of Americans using powers granted by Congress in the aftermath of the September 11, 2001 terrorist attacks.

In 2013, then-NSA contractor Edward Snowden leaked thousands of classified documents to journalists detailing widescale NSA surveillance around the world.

Klein’s death was confirmed by the Electronic Frontier Foundation, the San Francisco-based digital rights group who Klein turned to, and which went on to sue the federal government following Klein’s disclosures. The case was eventually dismissed.