Thomas Tamm, whose suburban Washington home was searched by federal agents last year, told the magazine he leaked the existence of the secret program to The New York Times 18 months before the newspaper broke the story.

"I thought this was something the other branches of the government and the public ought to know about. So they could decide: do they want this massive spying program to be taking place?" Tamm told Newsweek in what the magazine said were a series of recent interviews that he granted against the advice of his lawyers.

"If somebody were to say, who am I to do that? I would say, 'I had taken an oath to uphold the Constitution.' It's stunning that somebody higher up the chain of command didn't speak up," the magazine quoted Tamm as saying.

Tamm, 56, told the magazine he called the Times from a subway station pay phone in Washington.

In December 2005 the Times published a story exposing the Bush administration's warrantless wiretapping program to eavesdrop on international phone calls and e-mails of U.S. residents without court warrants.

The story cited multiple anonymous sources. Newsweek said the Times reporters who wrote the story refused to say whether Tamm was one of them.

The eavesdropping had been conducted without public knowledge and without any court approval. It has since been put under the authority of the Foreign Intelligence Surveillance Court.

Tamm, who left the Justice Department in 2006, had worked in the department's Office of Intelligence Policy and Review, a secretive unit that oversees surveillance of terrorist and espionage targets, according to Newsweek.

He told the magazine he has since struggled to make a living in private practice. Tamm has not been charged with any crimes, though the magazine said his friends and relatives have been questioned by federal agents.