The National Security Agency and the FBI are tapping directly into the central servers of nine leading U.S. Internet companies, allowing investigators to examine e-mails, photos and other documents that can be used to track people and their contacts over time, The Washington Post reported on Thursday.

The highly classified anti-terrorism program, code-named PRISM, had not been disclosed publicly before. A U.S. government source who was not authorized to comment publicly on the program confirmed its existence to Reuters late Thursday.

The program's participants, the Post said, include most of the dominant global players of Silicon Valley: Microsoft, Yahoo, Google, Facebook, PalTalk, AOL, Skype, YouTube and Apple.

The report came on the same day that another newspaper, Britain's Guardian, revealed that the U.S. government is collecting telephone records of millions of Americans as part of U.S. counterterrorism efforts.

In Washington, the Guardian report fueled an ongoing debate over whether the U.S. government is violating citizens' privacy rights while trying to protect them from attacks. That debate is sure to escalate with the Post's report, which said the NSA and FBI are extracting audio, video, photographs, e-mails, documents and connection logs to build a database involving trillions of communications by Americans.

In statements released late Thursday, Google, Apple, Yahoo and Facebook denied that the government had "direct access" to their central servers.

"Google cares deeply about the security of our users' data. We disclose user data to government in accordance with the law, and we review all such requests carefully. From time to time, people allege that we have created a government 'back door' into our systems, but Google does not have a 'back door' for the government to access private user data," the company said in a statement.

"Protecting the privacy of our users and their data is a top priority for Facebook. We do not provide any government organization with direct access to Facebook servers," Facebook's Chief Security Officer Joe Sullivan said in a statement.

"When Facebook is asked for data or information about specific individuals, we carefully scrutinize any such request for compliance with all applicable laws, and provide information only to the extent required by law."

The Post said it planned to released a statement late on Thursday on the article and the companies' comments.

The program was established under Republican President George W. Bush in 2007 and has grown exponentially since then during President Barack Obama's administration, the Post article said.

The Post said it obtained copies of briefing slides describing the Silicon Valley operation that were intended for senior analysts in the NSA's Signals Intelligence Directorate.

The slides described the new tool as the most prolific contributor to the President's Daily Brief, which cited PRISM data in 1,477 articles last year.

The technology companies participate knowingly in PRISM operations and are listed on one NSA slide in the order in which they became part of the program, starting with Microsoft in September 2007 and ending with Apple in October 2012.

The Post said it obtained the PowerPoint slides about PRISM and supporting materials from a career intelligence officer who wanted to expose what he believes to be a gross intrusion on privacy. "They quite literally can watch your ideas form as you type," the officer said.

(Additional reporting by Gerry Shih and Alexei Oreskovic; Editing by David Lindsey and Jim Loney)