An ex-CIA employee working as a contractor at the US National Security Agency said he was the source who leaked details of a top secret US surveillance programme, acting out of conscience to protect “basic liberties for people around the world.”

Holed up in a hotel room in Hong Kong, Edward Snowden, 29, 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.

His whereabouts were not immediately known yesterday, but staff at a luxury hotel in Hong Kong said that Snowden had checked out at noon.

Snowden, a former technical assistant at the CIA, said he had been working at the super-secret NSA as an employee of contractor Booz Allen. He said he decided to leak information after becoming disenchanted with President Barack Obama, who he said had continued the policies of predecessor George W. Bush.

“I don’t want to live in a society that does these sort of things... I do not want to live in a world where everything I do and say is recorded. That is not something I am willing to support or live under,” he told The Guardian newspaper, which published a video interview with him on its website. The interview was dated June 6.

Both The Guardian and The Washington Post said last week that US security services had monitored data about phone calls from Verizon and Internet data from large companies such as Google and Facebook.

In naming Snowden on Sunday, the newspapers said he had sought to be identified.

“The NSA has built an infrastructure that allows it to intercept almost everything,” Snowden said in explaining his actions.

“With this capability, the vast majority of human communications are automatically ingested without targeting. If I wanted to see your e-mails or your wife’s phone, all I have to do is use intercepts. I can get your e-mails, passwords, phone records, credit cards,” he said.

The Guardian said Snowden had been working at the NSA for four years as a contractor for outside companies.

Three weeks ago, he copied the secret documents at the NSA office in Hawaii and told his supervisor he needed “a couple of weeks” off for treatment for epilepsy, the paper said. On May 20 he flew to Hong Kong.

A spokesman for the Director of National Intelligence would not comment about Snowden but said the intelligence community was reviewing damage done by the recent leaks.

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