Mark Klein, a former AT&T technician who became a whistleblower, has passed away at the age of 79, after revealing the U.S. government’s mass surveillance program.

In 2006, Klein came forward with documents that exposed the NSA’s secret operation in an AT&T facility in San Francisco, where they were tapping into the internet’s backbone.

The notorious Room 641A was the site of this covert operation, where optical splitting wiretaps created a duplicate of all internet traffic, which was then sent to the NSA.

Klein’s revelations confirmed that the U.S. government was leveraging powers granted by Congress after the 9/11 attacks to access the internet data of millions of Americans.

Seven years later, in 2013, Edward Snowden, a former NSA contractor, leaked a vast number of classified documents to journalists, revealing the NSA’s widespread surveillance activities worldwide.

The Electronic Frontier Foundation, a San Francisco-based digital rights organization that Klein had turned to, confirmed Klein’s passing. The organization had filed a lawsuit against the federal government following Klein’s disclosures, but it was ultimately dismissed.