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 and 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".
His leaks led to revelations that the US is systematically seizing vast amounts of phone and web data.
The programme, known as Prism, is run by the US National Security Agency (NSA).
The Office of the Director of National Intelligence gave details of the programme last week after Mr Snowden's leaks led to a series of articles in the Guardian and Washington Post newspapers.
According to the office's statement, Prism is simply an internal computer system, and not a data-mining programme.
However, such data seizures could break the laws of other countries, and could also break US law if they accidentally capture communications of US citizens.
Hong Kong's broadcaster RTHK said Mr Snowden checked out of the Mira hotel in Kowloon on Monday, and Reuters news agency quoted hotel staff as saying that he had left at noon.
Ewen MacAskill, one of the Guardian journalists who broke the story, told the BBC he believed Mr Snowden was still in Hong Kong.
It is believed the US is pursuing a criminal investigation, but no extradition request has yet been filed.
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.
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.
Mr Snowden's revelations have led to allegations that the UK's electronic surveillance agency, GCHQ, used the US system to spy on British citizens.
Foreign Secretary William Hague cancelled a trip to Washington to address the UK parliament on Monday and deny the claims.
The journalists involved in the story were first contacted by Mr Snowden at the start of the year.
Filmmaker Laura Poitras told Salon Magazine how Mr Snowden sent her an email saying: "I want to get your encryption key and let's get on a secure channel.
"I have some information in the intelligence community, and it won't be a waste of your time."
Ms Poitras ultimately filmed the interview with two Guardian reporters.
Mr Snowden told the journalists: "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."
Booz Allen Hamilton confirmed in a statement 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.
The first of the leaks came out on Wednesday night, when the Guardian reported a US secret court had 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).
Under the Prism system, officials apply to the Foreign Intelligence Surveillance Act Court (Fisa) to gain access to communications.
Officials are obliged to show the Fisa court that any "target" is outside of the US, and there is a "foreign intelligence purpose" for the seizure, such as terrorism or nuclear proliferation.
However, details of such targets and subsequent requests made to US technology companies are secret.
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 denied giving the US government "direct access" to their servers, and said they had never heard of the Prism programme.
But in separate statements the firms variously said they complied with lawful requests to supply information on an individual basis.
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, saying that nobody was listening to calls between American citizens.