"As to the individual who has admitted to making these disclosures, he is the subject of an ongoing criminal investigation," FBI Director Robert Mueller told the House Judiciary Committee.
"These disclosures have caused significant harm to our nation and to our safety. We are taking all necessary steps to hold the person responsible for these disclosures," he said.
The FBI chief's comments offered the first explicit confirmation that the US government was pursuing Snowden, the 29-year-old American IT specialist who has admitted to leaking information about far-reaching surveillance programs.
Mueller defended the collection of American phone records and Internet traffic as legal programs that were approved by a judge and were in accordance with the Constitution.
He added that the government was determined to safeguard privacy rights and civil liberties even as it seeks to prevent possible terrorist attacks on the United States.
