In a wide-ranging interview with Wired magazine over several days from Moscow, Edward Snowden said he had been troubled for years by the activities of the National Security Agency but that national intelligence chief James Clapper's testimony prompted him to act.
The former NSA contractor said he made his decision after reading in March 2013 about Clapper telling a Senate committee that the NSA does "not wittingly" collect information on millions of Americans.
"I think I was reading it in the paper the next day, talking to coworkers, saying, can you believe this...?"
Snowden told journalist James Bamford he had been troubled by other discoveries, including NSA spying on the pornography-viewing habits of political radicals.
"It's much like how the FBI tried to use Martin Luther King's infidelity to talk him into killing himself," he said. "We said those kinds of things were inappropriate back in the '60s. Why are we doing that now?"
Snowden also was disturbed by the NSA's effort to massively speed up data collection with a secret data storage facility in Bluffdale, Utah, which scanned billions of phone calls, faxes, emails, computer-to-computer data transfers, and text messages from around the world.
He put off his plan to leak NSA secrets at the time of the election of President Barack Obama, hoping for a more open government. But he became disenchanted with the president, and by 2013 was ready to spill the secrets he had acquired.
After Clapper's testimony to Congress, Snowden said his colleagues did not appear shocked, but he was concerned he was getting in too deep in an "evil" system.
"It's like the boiling frog," he says, in a reference to the fable of a frog placed in cold water who fails to realize the water is heating up gradually, until it is too late.
"You get exposed to a little bit of evil, a little bit of rule-breaking, a little bit of dishonesty, a little bit of deceptiveness, a little bit of disservice to the public interest, and you can brush it off, you can come to justify it," he told the magazine.