By David Ingram and Patricia Zengerle
WASHINGTON (Reuters) - FBI Director Robert Mueller said on Thursday that authorities would move aggressively to track down Edward Snowden and hold him accountable for leaking the details of extensive and top-secret U.S. surveillance efforts.
Mueller confirmed that a criminal investigation had been launched into the leaks and said public reports about the National Security Agency's efforts to monitor Internet and phone data had hurt U.S. national security.
"We are taking all necessary steps to hold the person responsible for these disclosures," Mueller told the House of Representatives Judiciary Committee, without naming Snowden.
"These disclosures have caused significant harm to our nation and to our safety," he said.
Snowden, a former NSA contractor, has acknowledged he was the source of reports last week in Britain's Guardian newspaper and The Washington Post about the agency's monitoring of phone and Internet data at big companies such as Google Inc and Facebook Inc.
Snowden, who traveled to Hong Kong before the programs were made public, said in a newspaper interview published on Wednesday that he planned to stay in the China-ruled city to fight any effort to bring him back to the United States to face charges.
Mueller joined President Barack Obama and other administration members in defending the programs as a crucial tool in preventing possible attacks. He said making the details public could force a switch in tactics by potential terrorists.
"We're going to ... lose our ability to get their communications. We are going to be exceptionally vulnerable," Mueller said. "Let nobody be misled in this. This hurts national security."
General Keith Alexander, head of the NSA, told Congress on Wednesday the programs had helped disrupt dozens of possible terrorist attacks. U.S. Senator Dianne Feinstein, the Democrat who heads the Senate Intelligence Committee, said details would be made public on Monday.
The White House said the program had led to the 2009 arrest of a Chicago man who was planning to bomb a Danish newspaper that had published cartoons of the Prophet Mohammad.
It also confirmed assertions by U.S. officials and members of Congress that electronic eavesdropping by the NSA had helped foil a plot by Islamist militants to bomb the New York subway system in 2009.
The revelations about the surveillance renewed a political debate about the proper balance between privacy rights and national security, and some lawmakers and advocacy groups have called for tighter supervision of the programs.
"It's my hope that over the coming weeks the members of this Judiciary Committee can come together and conduct meaningful oversight of these programs" including possibly passing new legislation, said Representative John Conyers of Michigan, the ranking Democrat on the House panel.
Feinstein told reporters Congress would consider legislation to limit access to some classified information by contractors.
"We will certainly have legislation which will limit or prevent contractors from handling highly classified technical data," she said after a closed-door briefing on the program by intelligence and law enforcement officials.
Republican Senator Rand Paul of Kentucky, a conservative Tea Party favorite and potential 2016 presidential contender, encouraged Americans to bring a class-action lawsuit against the U.S. government for the surveillance.
"Americans are rightly concerned about having all of their phone records collected and monitored all the time," Paul said at a news conference that brought together the American Civil Liberties Union and Republicans including U.S. Representatives Louie Gohmert and Justin Amash.
So far, 250,000 people have signed up on Paul's political action committee's website to take part in a lawsuit challenging the constitutionality of the programs, although Paul admitted he did not know how the lawsuit would be achieved.
Mueller said the programs did not go too far in snooping on Americans. He said the programs are limited to collecting "the fact of a telephone call, the numbers called, and the time and length of those calls" and have "no authority to get content."
"And there are cases where that has been instrumental in identifying individuals who sought to harm our country," he said.
The leaders of the House Intelligence Committee said the programs respected privacy rights and had saved dozens of lives. U.S. Representative Mike Rogers of Michigan, chairman of the panel, questioned Snowden's motives.
"There is a long list of questions we have to get answered about does he have a relationship with a foreign government, and is there more to this story? I think there's questions that have not been answered on that," Rogers said.
Despite frequent administration claims that Congress had been fully briefed, some lawmakers were taken aback at the extent of the surveillance. U.S. Representative Loretta Sanchez, a California Democrat, said she and other members of Congress were "clearly surprised" at the reports on the NSA surveillance.
"It is in the NSA's best interest to declassify some of the processes the agency undertakes so that we as a country can engage in an educated, thoughtful debate on a balance between our security and civil liberties," she said.
(Additional reporting by Mark Hosenball, Matt Haldane, Thomas Ferraro; Writing by John Whitesides; Editing by Karey Van Hall and Doina Chiacu)