The US administration hit back Tuesday after being accused of undermining press freedom by seizing reporters' phone records, insisting officials are probing a dangerous leak of state secrets.
Attorney General Eric Holder told a news conference that investigators had secretly obtained telephone logs from a US news agency while seeking the source of 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 declared.
The comments came as the US government faced a barrage of criticism over the Justice Department's decision to seize two months of phone records from the news operations of the Associated Press.
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 in 2012 to detonate a bomb on an airplane bound for the United States.
Holder noted that he had recused himself from the probe because he was interviewed by the FBI about unauthorized disclosures in the matter.
A Justice Department statement said that since Holder's recusal in June 2012, the investigation "has been conducted by the FBI under the direction of the US Attorney and the supervision of the deputy attorney general."
"The decision to seek media toll records in this investigation was made by the deputy attorney general consistent with department regulations and policies," the statement said.
The White House, meanwhile, sought to deflect criticism that it was targeting the news media in its war on leaks of classified or secret information.
President Barack Obama's spokesman Jay Carney said the White House was "not involved" in the decisions to seek AP records, noting that the Justice Department operates independently.
"I can't comment on the specifics of that, but I can tell you that the president feels strongly that we need... the press to be able to be unfettered in its pursuit of investigative journalism," Carney told reporters.
"And he is also mindful of the need for secret and classified information to remain secret and classified in order to protect our national security interests."
Carney added that Obama has allowed the investigation to proceed and that "it would be wholly inappropriate for the president to involve himself in a criminal investigation that... at least as reported, involves leaks of information from the administration."
The AP protested the seizure in a letter to Holder saying "there can be no possible justification for such an overbroad collection" of phone records.
The revelation also brought a flurry of criticism of the administration for what critics called an unprecedented assault on press freedom.
Trevor Timm of the Freedom of the Press Foundation said "it's time to stop looking at all of these leak investigations and prosecutions as ancillary to press freedom; they are a direct attack on it.
"This should be an important wake-up call for journalists."
Other analysts noted that the Obama administration had already gained a reputation for aggressively pursuing leaks of government secrets, and had gone a step further with the latest seizure.
"It's surprising and concerning to me that they would sweep so broadly in the search of AP phone records," David Pozen, a specialist in constitutional and national security law at Columbia University, told AFP.
"It certainly seems like an aggressive interpretation of the Justice Department's subpoena policy which has been in place since the 1970s to ensure that prosecutors proceed cautiously and narrowly when engaging the news media in such matters."