The US government claims it was only trying to protect American lives when it took the drastic step of seizing journalists' phone records in a probe of what it calls a major security breach.
President Barack Obama's administration, embattled on several fronts, now faces a barrage of criticism that the step undermined freedom of the press.
Attorney General Eric Holder hit back, defending the action in which the Justice Department secretly took two months of telephone logs from news operations of the Associated Press.
He said this was done as part of a probe into a security breach which had put the American people at risk.
"I've been a prosecutor since 1976. And I have to say that this is among, if not the most serious ... a ... very, very serious leak," Holder said.
"That's not hyperbole. Puts the American people at risk. And trying to determine who is responsible for that, I think, required very aggressive action," he said.
The investigators' action is believed to be linked to a probe into a story on a foiled terror plot, which they suspect contained leaked information.
The AP said its story disclosed details of a CIA operation in Yemen that stopped an al-Qaeda plot last year to detonate a bomb on a plane bound for the United States.
Holder said he had recused himself from the probe because he was interviewed by the FBI about unauthorised disclosures in the matter.