The U.S. Attorney's office said, of its investigation into AP, "We take seriously our obligations to follow all applicable laws, federal regulations, and Department of Justice policies when issuing subpoenas for phone records of media organizations. Those regulations require us to make every reasonable effort to obtain information through alternative means before even considering a subpoena for the phone records of a member of the media. We must notify the media organization in advance unless doing so would pose a substantial threat to the integrity of the investigation."
Holder called the information given to the AP, "a very, very serious leak ... among the most serious" he had seen in his 37-year career.
"It put the American people at risk," he said. "That is not hyperbole."
ABC News' Jack Date contributed to this report.