Mukasey told the House Judiciary Committee the Justice Department could not open a criminal investigation now because the controversial interrogation technique was part of an approved program at the time it was used, The Washington Post reported.
"Waterboarding, because it was authorized to be part of a program ... cannot possibly be the subject of a Justice Department investigation," Mukasey responded to questions from Rep. John Conyers Jr., D-Mich., committee chair.
In another committee hearing Thursday, CIA Director Michael Hayden said waterboarding, which simulates drowning, was necessary but probably illegal.
Hayden told the House Intelligence Committee the technique could be used if "an unlawful combatant is possessing information that would help us prevent catastrophic loss of life of Americans or their allies," CNN said.
"In my own view, the view of my lawyers and the Department of Justice, it is not certain that that technique would be considered lawful under current statute," he said.
Hayden told a Senate committee Tuesday the CIA used waterboarding on three al-Qaida detainees in 2002 and 2003.