The beleaguered 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 on Tuesday, 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 declared.
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 said 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 White House sought to deflect criticism that it was targeting the news media in its war on leaks.
Obama spokesman Jay Carney said the White House was not involved in the decision to seek AP records and the Justice Department operates independently.
"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," Carney said.
The AP protested the seizure Monday in a letter to Holder saying "there can be no possible justification for such an overbroad collection."
In a reply to the AP, Deputy Attorney General James Cole said the probe into leaked "classified information" began last year and warned "such disclosures can risk lives and cause grave harm to the security of all Americans."
Cole said agency rules require that other steps be exhausted before the seizure of phone records, and the action was taken only after "conducting over 550 interviews and reviewing tens of thousands of documents."
He said the subpoenas were "limited to a reasonable period of time and did not seek the content of any calls."
In a new response to Cole's letter, the AP's chief executive, Gary Pruitt, disputed this, insisted the inquiry had not been "narrowly drawn" and said that more than 100 journalists work in the locations targeted.
"The White House had said there was no credible threat to the American people in May of 2012. The AP story suggested otherwise, and we felt that was important information and the public deserved to know it," Pruitt said.
The revelation of the seizure brought a flurry of criticism of what critics called an unprecedented assault on press freedom.
Senate Democratic leader Harry Reid, a leading member of Obama's Democratic Party, told reporters: "I have trouble defending what the Justice Department did... it's inexcusable."
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.
"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."