Advice for Snowden from a man who knows: Always check six

'And always check six, as we said when I used to be a flyer in the Air Force. Always make sure you know what's behind you'

Thomas Drake is one of the few people who understands from personal experience what the future may hold for Edward Snowden, the 29-year-old former NSA contractor who exposed the US government's top secret phone and Internet surveillance programmes.

His advice for Snowden: "Be lawyered up to the max and find a place where it's going to be that much more difficult for the US to make arrangements for his return," Drake said. "And always check six, as we said when I used to be a flyer in the Air Force. Always make sure you know what's behind you."

Drake, a 56-year-old former intelligence official at the National Security Agency, was prosecuted under the Espionage Act in 2010 for allegedly revealing classified information about the agency's sweeping warrantless wire-tapping programme. The government later dropped all but a misdemeanour charge.

"For me this is a dejà vu," Drake said, adding that Snowden's previous comfortable life was over. "When you offer up information about the dark side of the surveillance state they don't take too kindly to it," he said. "They want to stay in the shadows."

Drake, one of six people indicted for leaking secret information since US President Barack Obama took office in 2009, said the FBI investigated him because it believed he was the source of a New York Times story published in December 2005 that first revealed the NSA's wire-tapping programme. He says he was not the source of that information, and 10 felony counts against him were dropped when he pleaded guilty to a misdemeanor charge of mishandling government information.

In a series of interviews over the past week, he described the experience of coming under investigation.

"My life was turned upside down and inside out," said Drake, who now earns an hourly wage as a technical expert at an Apple store. "I know what it's like to live in a surveillance state because the surveillance state was on me, riding me, for so many years. They obviously wanted to do me in. It was relentless. I wouldn't want any American to go through it."

Snowden, who worked for three months for Booz Allen Hamilton and was contracted out as a systems administrator to the NSA Threat Operations Center in Hawaii, disclosed this weekend that he was the source of last week's reports in The Guardian and The Washington Post, saying he acted out of conscience to protect "basic liberties for people around the world".

Snowden said he had thought long and hard before publicising details of an NSA programme code-named PRISM, saying he had done so because he felt the US was building an unaccountable and secret espionage machine that spied on every American.