Mark Klein, a whistleblower who exposed a National Security Agency program of mass government spying on American internet traffic in the 2000s, has died. He was 79 years old.

His brother, Larry Klein, told PBS News that Mark died March 8 in Oakland, California. Klein had been diagnosed with Parkinson’s disease and, later, pancreatic cancer, his brother said.

The Electronic Frontier Foundation, who worked with Klein to expose the spying program, said in a statement that he “told the truth at great personal risk to himself and his family.”

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“I could never imagine being in that kind of a situation,” Larry said of his brother’s efforts. “I’m not sure I would have had the courage to continue through with this. I was so proud of him.”

Klein was a technician at AT&T in San Francisco when he discovered that the NSA was using a splitter to intercept, monitor and copy all internet traffic and other data and deliver it to a secure room inside AT&T’s office. He found designs for the space, known as Room 641A, and other documents that detailed the technology and the existence of the program, as well as outlined AT&T’s cooperation with the spy agency. He determined that the program existed in other U.S. cities. The NSA program came on the heels of the Patriot Act, which created the Department of Homeland Security and gave U.S. defense and spy agencies wide latitude in the wake of the Sept....

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