President Barack Obama's administration is likely to open a criminal investigation into the leaking of highly classified documents that revealed the secret surveillance of Americans' telephone and email traffic, US officials said on Friday.

The law enforcement and security officials, who were not authorized to speak publicly, said the agencies that normally conduct such investigations, including the FBI and Justice Department, were expecting a probe into the leaks to a British and an American newspaper.
 
Such investigations typically begin after an agency that believes its secrets have been leaked without authorization files a complaint with the Justice Department.

It was unclear on Friday whether a complaint had been submitted by the publicity-shy National Security Agency, which was most directly involved in the collection of trillions of telephone and email communications.

However, one US official with knowledge of the situation said that given the extent and sensitivity of the recent leaks, federal law may compel officials to open an investigation.

A criminal probe would represent another turn in the Obama administration's battle against national security leaks. This effort has been under scrutiny lately because of a Justice Department investigation that has involved searches of the phone records of Associated Press journalists and a Fox News reporter.

Leaks to media outlets this week have revealed a government campaign of domestic surveillance going far beyond anything that had been acknowledged previously.

Late on Wednesday, Britain's Guardian newspaper published what US officials later acknowledged was an order, approved by the secretive US Foreign Intelligence Surveillance Court, requiring a subsidiary of Verizon Communications to give the NSA raw data showing phone calls made from numbers within the United States and from US numbers to those overseas.

The data did not include the identities of people who made the calls or the contents of the calls.

On Thursday, the Guardian and the Washington Post published slides from a secret NSA powerpoint presentation that described how the agency gathered masses of email data from prominent Internet firms, including Google, Facebook and Apple under a Top-Secret program called PRISM.

Some of the companies denied that the NSA and FBI had "direct access" to their central servers, as the Post reported.

On Friday, for example, Facebook founder and Chief Executive Mark Zuckerberg said his company "is not and has never been part of any program to give the US or any other government direct access to our servers."

"We have never received a blanket request or court order from any government agency asking for information or metadata in bulk, like the one Verizon reportedly received," Zuckerberg said. "And if we did, we would fight it aggressively. We hadn't even heard of PRISM" before Thursday, he said.

James Clapper, the director of US national intelligence, condemned the leaks and asserted that the news articles about PRISM contained "numerous inaccuracies."

Journalists involved in The Guardian and Washington Post articles have reported in depth on WikiLeaks, the website known for publishing secret US government documents.

The Post report on the PRISM program was co-written by Laura Poitras, a filmmaker who has been working on a documentary on WikiLeaks, with the cooperation of its founder Julian Assange, and who last year made a short film about Bill Binney, a former NSA employee who became a whistleblowing critic of the agency.

Last year, the web magazine Salon published a lengthy article by the author of the Guardian report, Glenn Greenwald, accusing US authorities of harassing Poitras when she left and re-entered the United States. Greenwald also has written frequently about Assange.

The Guardian and Post stories appeared in the same week that US Army Private First Class Bradley Manning went on trial in Maryland accused of leaking hundreds of thousands of classified documents to WikiLeaks.

In an email to Reuters on Friday, Poitras rejected the notion that the trial had any impact on the timing of her story.

"I am fully aware we are living in a political climate where national security reporting is being targeted by the government, however, I don't think fear should stop us from reporting these stories," Poitras wrote.

"To suggest that the timing of the NSA PRISM story is linked in any way to other events or stories I'm following is simply wrong. Like any journalist, I have many contacts and follow multiple stories."

Kris Coratti, a Washington Post spokeswoman, said the timing of the paper's publication of Poitras' story had nothing to do with Manning's trial and that Assange had played no role in arranging or encouraging the story.

Greenwald did not respond to emailed requests for comment. The Guardian's editor-in-chief, Alan Rusbridger, declined to comment.-Reuters