Room 641A, which housed the “Big Brother machine,” inside AT&T’s Folsom Street building in San Francisco, California.

On January 20, 2006, the front doorbell rang at the Electronic Frontier Foundation’s offices on Shotwell Street in the Mission District of San Francisco. At the time, Shotwell Street wasn’t the glamorous part of the Mission. Our offices sat between two auto repair shops, across the street from a utility substation.

[I]t was with friendliness but some caution that our executive director, Shari Steele, answered the bell.

“Do you folks care about privacy?” the guy asked. He was in a tan trench coat, looked to be in his early 60s, with gray hair, intense eyes, and a raspy voice.

“Why yes, we do,” Shari answered.

“Then I have some information for you. I am a retired AT&T technician. I know how the NSA is tapping into the internet at an AT&T facility downtown.”

“Well, come on in.”

Shari found EFF attorney Kevin Bankston in his tiny office. They talked for a long time. After the man left, Kevin and Lee Tien, another EFF attorney, burst into my office.

“This guy named Mark Klein, who just came to the door, has something,” Kevin said, with more excitement than I had seen from him in a long time. I was immediately intrigued, but what they told me blew past my highest expectations. Mark had presented us with unequivocal evidence that the National Security Agency was engaged in mass, untargeted spying in the U.S. by tapping into the internet backbone. And it was doing this from an AT&T building just a short distance from our offices.