US Attorney General Eric Holder told reporters on Tuesday the leak prompting his department to monitor phone calls made by Associated Press reporters was "very serious" but declined further comment.
Holder said he recused himself from the case because of his frequent contact with the media, meaning Deputy Attorney General James Cole was in charge of the investigation.
The Justice Department informed the Associated Press of the breach by letter on Friday, and it was immediately protested as a violation of the media's "constitutional rights to gather and report the news" in a letter written by AP President Gary Pruitt on Monday.
According to the AP, the Justice Department secretly monitored records for 20 phone lines used by AP journalists over two months, noting the length of the conversations as well as the outgoing and incoming numbers at bureaus in several major US cities, including Washington, DC.
Cole responded to the AP by letter on Tuesday, acknowledging that the department is required to "negotiate" with media organizations in such investigations "unless doing so would pose a substantial threat to the integrity of the investigation."
Cole did not identify the nature of the probe and refused to disclose which AP records the DOJ had accessed, saying doing so would jeopardize the extensive ongoing investigation.
Holder described the leak under investigation as one of the most threatening he'd seen since he started working at the department in 1976.
"That is not hyperbole," Holder said. "It put the American people at risk."
More from GlobalPost: AP: Justice Dept monitored phone calls in 'unprecedented intrusion'
The AP phone records, believed collected during April and May 2012, could be related to an investigation into a May 7, 2012 AP story that included leaked information on a foiled terrorist plot and a CIA operation in Yemen, said the AP.
The Justice Department has not clarified the matter, but BBC News reported that the phone lines of five reporters and an editor who worked on that story were allegedly among those monitored by the government.
An AP representative confirmed to The Huffington Post on Tuesday that Washington bureau chief Sally Buzbee was among those targeted, but would not say whether or not she was involved in the May 7 story.
Holder said he could not provide any other details on the AP phone probe, deferring to Cole. That hasn't kept him out of the fire, though -- Republican National Committee Chairman Reince Priebus said Tuesday he should resign over the AP affair.
"Freedom of the press is an essential right in a free society. The First Amendment doesn't request the federal government to respect it; it demands it," Priebus wrote, according to CNN. "Attorney General Eric Holder, in permitting the Justice Department to issue secret subpoenas to spy on Associated Press reporters, has trampled on the First Amendment and failed in his sworn duty to uphold the Constitution."
The incident has raised questions about the government's relationship with the Fourth Estate.
The Associated Press Media Editors Association slammed the Obama administration for the move, calling it part of a "continuing witch hunt for leaks and whistleblowers."
The White House denied this on Tuesday, with spokesman Jay Carney telling reporters: "The president believes that the press as a rule needs to have an unfettered ability to pursue investigative journalism," according to Reuters.
All this has lead to a lively discussion on social media:[View the story "AP phone probe prompts freedom of speech debate on Twitter" on Storify]