Holder said he had removed himself from the matter because of congressional testimony he had given and his dealings with the news media.
The Justice Department secretly obtained two months of telephone records of AP reporters and editors.
Holder said federal prosecutors are looking into the matter.
"This was a very serious leak, a very grave leak" that "put the American people at risk," Holder told reporters at a news conference Tuesday.
Holder has assigned Deputy Attorney General Jim Cole to handle the phone records case.
The Justice Department secretly obtained two months of telephone records of reporters and editors for The AP in what the news cooperative's top executive called a massive and unprecedented intrusion into how news organizations gather the news.
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