FBI Director Robert Mueller apologized Friday to New York Times editor Bill Keller and Washington Post editor Leonard Downie Jr. for the 2004 incidents, in which agents used so-called national security letters to obtain the reporters' phone records, the Times reported.

FBI officials told the newspaper the incidents were discovered as the U.S. Justice Department's inspector general conducts an investigation into improper use of the national security letters, which are supposed to be used only in emergencies to prevent terrorist actions.

"The FBI is committed to protecting the news media consistent with the First Amendment and Department of Justice policies, and we very much regret that this situation occurred," Valerie Caproni, general counsel for the bureau, wrote in a letter to Keller Friday, the Times said.