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An ex-CIA employee who leaked details of US top-secret phone and internet surveillance has disappeared from his hotel in Hong Kong.
Edward Snowden, 29, checked out from his hotel on Monday. His whereabouts are unknown, but he is believed to be still in Hong Kong.
Earlier, he said he had an "obligation to help free people from oppression".
It emerged last week that US agencies were gathering millions of phone records and monitoring internet data.
A spokesman for the US Office of the Director of National Intelligence said the case had been referred to the Department of Justice as a criminal matter.
Meanwhile a petition posted on the White House website, calling for Mr Snowden's immediate pardon, has gathered more than 30,000 signatures.
However an opinion poll commissioned by the Washington Post suggests a majority of Americans think government monitoring of phone records is acceptable if the aim is to fight terrorism.
Transatlantic fallout
Hong Kong's broadcaster RTHK said Mr Snowden checked out of the Mira hotel on Monday.
Reuters news agency quoted hotel staff as saying that he had left at noon.
Ewen MacAskill, a Guardian journalist, told the BBC he believed Mr Snowden was still in Hong Kong.
The Chinese territory has an extradition treaty with the US, although analysts say any attempts to bring Mr Snowden to America may take months and could be blocked by Beijing.
Mr Snowden was revealed as the source of the leaks at his own request by the UK's Guardian newspaper.
He is believed to have arrived in Hong Kong on 20 May. A standard visa on arrival in the territory for a US citizen lasts for 90 days.
His revelations have caused transatlantic political fallout, amid allegations that the UK's electronic surveillance agency, GCHQ, used the US system to snoop on British citizens.
Foreign Secretary William Hague cancelled a trip to Washington to address the UK parliament on Monday and deny the claims.
Mr Snowden is described by the Guardian as an ex-CIA technical assistant, currently employed by Booz Allen Hamilton, a defence contractor for the US National Security Agency (NSA).
He told the newspaper: "The NSA has built an infrastructure that allows it to intercept almost everything. With this capability, the vast majority of human communications are automatically ingested without targeting.
"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."
Mr Snowden said he did not believe he had committed a crime: "We have seen enough criminality on the part of government. It is hypocritical to make this allegation against me."
But he admitted that he could end up in jail and feared for people who knew him.
'Core values'
In a statement, Booz Allen Hamilton confirmed Mr Snowden had been an employee for less than three months.
"If accurate, this action represents a grave violation of the code of conduct and core values of our firm," the statement said.
At a daily press briefing on Monday, White House press secretary Jay Carney said he could not comment on the Snowden case, citing an ongoing investigation.
The first of the leaks came out on Wednesday night, when the Guardian reported a US secret court ordered phone company Verizon to hand over to the NSA millions of records on telephone call "metadata".
The metadata include the numbers of both phones on a call, its duration, time, date and location (for mobiles, determined by which mobile signal towers relayed the call or text).
On Thursday, the Washington Post and Guardian said the NSA tapped directly into the servers of nine internet firms including Facebook, Google, Microsoft and Yahoo to track online communication in a programme known as Prism.
All the internet companies deny giving the US government access to their servers.
Prism is said to give the NSA and FBI access to emails, web chats and other communications directly from the servers of major US internet companies.
The data is used to track foreign nationals suspected of terrorism or spying. The NSA is also collecting the telephone records of American customers, but said it is not recording the content of their calls.
US director of national intelligence James Clapper's office said information gathered under Prism was obtained with the approval of the secret Foreign Intelligence Surveillance Act Court (Fisa).
Prism was authorised under changes to US surveillance laws passed under President George W Bush, and renewed last year under Barack Obama.
Mr Obama has defended the surveillance programmes, assuring Americans that nobody was listening to their calls.