Edward Snowden, the former National Security Agency contractor who exposed mass cybersurveillance by American and British spy agencies, says the US government fears the most damaging leaks are yet to come.
"I think they think there's a smoking gun in there that would be the death of them all politically," Snowden said in a new interview with Wired, a monthly US technology magazine.
Speaking from Russia where he was granted temporary asylum in June 2013 after leaving Hong Kong, Snowden said the NSA's audit of what he took had wrongly included all the documents he had simply "touched", putting the total estimate at a staggering 1.7 million papers.
"The fact that the government's investigation failed - that they don't know what was taken and that they keep throwing out these ridiculous huge numbers - implies to me that somewhere in their damage assessment they must have seen something that was like, 'Holy s**t'.
"And they think it's still out there."
In the interview, Snowden recalls taking a flight from Hawaii - where he worked for NSA contractor Booz Allen Hamilton - to Hong Kong in May 2013 to reveal himself to reporters as the source of the leaks.
"I thought it was likely that society collectively would just shrug and move on," he told Wired.
But since the now-famous 12-minute video in which Snowden revealed himself, a global debate has emerged about the extent to which governments secretly collect data.
The article, viewed by some as part of a campaign to win over the hearts and minds of Americans, was illustrated with a series of portraits including the magazine's cover showing Snowden tightly grasping the US flag while gazing into the distance.
The 31-year-old has always maintained that he would like to go home, but only under the right circumstances, ones in which he could be guaranteed a fair trial.
"I told the government I'd volunteer for prison, as long as it served the right purpose," he said.
"I care more about the country than what happens to me. But we can't allow the law to become a political weapon or agree to scare people away from standing up for their rights, no matter how good the deal. I'm not going to be part of that."
The article was based on a series of interviews conducted by former NSA whistle-blower James Bamford, who spent three days with Snowden spread out over several weeks this June.
Bamford added to growing speculation that there was another whistle-blower exposing NSA secrets, pointing out that a number of NSA leaks had not been linked to Snowden.
In Hong Kong last year, Snowden revealed to the South China Morning Post that the NSA had been hacking computers in the city and in mainland China for years - and he reiterated this point in the Wired interview.
"It's no secret that we hack China very aggressively," Snowden said. "But we've crossed lines. We're hacking universities and hospitals and wholly civilian infrastructure rather than actual government targets and military targets. And that's a real concern."
Last week, Snowden was granted a permit to stay in Russia for three more years. He faces up to 30 years in jail if convicted under the US Espionage Act.