WASHINGTON (REUTERS) - A secret United Service (US) intelligence program to collect emails that is at the heart of an uproar over government surveillance helped foil an Islamist militant plot to bomb the New York City subway system in 2009, US government sources said on Friday.

The sources said Representative Mike Rogers, chairman of the House of Representatives Intelligence Committee, was talking about a plot hatched by Najibullah Zazi, an Afghan-born US resident, when he said on Thursday that such surveillance had helped thwart a significant terrorist plot in recent years.

President Barack Obama's administration is facing controversy after revelations of details of massive programs run by the National Security Agency (NSA) for collecting information from telephone and Internet companies.

The surveillance program that halted the Zazi plot was one that collected email data on foreign intelligence suspects, a US government source said.

The Washington Post and Britain's Guardian newspaper on Thursday published top-secret information from inside NSA that described how the agency gathered masses of email data from prominent Internet firms, including Google, Facebook and Apple under the PRISM program. Some of the companies denied that the NSA and FBI had "direct access" to their central servers.

On Friday, CBS News correspondent John Miller, a former US intelligence and FBI official, reported that US authorities had discovered the Zazi plot after running across an email sent to a rarely used Al-Qaeda address that was associated with a notorious bomb-maker based in Pakistan.

Mr Miller said authorities traced the sender of the email to a suburb of Denver.

At the time of Zazi's arrest, US authorities revealed that he had been tracked from Denver to New York, where, after a brief interlude during which US investigators lost track of him, he was arrested by the FBI.

In February 2010, Zazi pleaded guilty in federal court in Brooklyn, New York, to charges that included conspiracy to use weapons of mass destruction and providing material support to terrorists.