The Obama administration is likely to open an investigation into how top secret documents about a secret surveillance program were leaked to the public.

According to Reuters, national security officials say the Justice Department and the Federal Bureau of Investigation are expected to open a probe into how The Washington Post and The Guardian newspapers obtained documents detailing how the National Security Agency collects Internet and telephone information.

A U.S. official told the news service that federal law may compel the launch of an investigation.

In response to the reports, major Internet companies denied that they knew anything about the surveillance program, called PRISM.

"We hadn't even heard of PRISM before yesterday," Facebook CEO Mark Zuckerberg wrote on his Facebook page.

On Friday President Obama defended the surveillance saying it is necessary to national security and "nobody" is actually listening in on phone calls.

"I want to be very clear: Some of the hype that we've been hearing over the last day or so, nobody's listening to the content of people's phone calls," Obama said.

The Justice Department also recently began an investigation into a different leak to Associated Press journalists about a foiled terrorism plot. Justice Department officials say that leak jeopardized national security.