Attorney General Eric H. Holder Jr. said Tuesday that he recused himself from involvement in a Justice Department leak investigation that secretly acquired telephone records of Associated Press journalists.

But in response to questions at a news conference, he defended the department’s conduct in probing what he described as one of the damaging leaks he has seen.

In a letter to Holder and his deputy Tuesday, a media coalition rejected what it called “an overreaching dragnet for newsgathering materials,” demanded that the Justice Department destroy the phone records and called on Congress to pass a federal shield law. The Washington Post joined more than 50 other news organizations in endorsing the letter.

Holder said he testified in June 2012 that he was interviewed by the FBI in connection with the probe into a leak of classified information to the AP. “To avoid any potential appearance of a conflict of interest,” he said, “I recused myself from this matter.”

Since then, he said, the investigation has been conducted by the FBI under the direction of the U.S. attorney for the District of Columbia and the supervision of the deputy attorney general, James M. Cole.

“The decision to seek media toll records in this investigation was made by the Deputy Attorney General consistent with Department regulations and policies,” the Justice Department said in a statement shortly before Holder made his remarks.

Holder said in reply to questions that he does not know “all that went into the formulation of the subpoena” for the phone records, but that “this was a very serious leak — a very, very serious leak.” He said that since he became a prosecutor in 1976, “this is among the top two or three serious leaks that I’ve ever seen.” He added that “it put the American people at risk” and that “trying to determine who was responsible for that required very aggressive action.”

He said he was “confident that the people who are involved in this investigation ... followed all of the appropriate Justice Department regulations and did things according to DOJ rules.”

Holder spoke in a joint news conference with Health and Human Services Secretary Kathleen Sebelius to announce unrelated law enforcement actions by the government’s Medicare Fraud Strike Force.

In a sweeping and unusual move, the Justice Department secretly obtained two months’ worth of telephone records of journalists working for the Associated Press as part of a year-long investigation into the disclosure of classified information about a failed al-Qaeda plot last year. The records listed outgoing calls from more than 20 work and personal phone lines in April and May 2012, the news agency said. It said the number of employees who used those lines during that period is unknown but that more than 100 journalists work in the targeted offices.

Federal authorities obtained cellular, office and home telephone records of individual reporters and an editor, as well as records from AP general office numbers in Washington, New York and Hartford, Conn., and the main number for AP reporters covering Congress, AP President and Chief Executive Officer Gary B. Pruitt said Monday. He called the Justice Department’s actions a “massive and unprecedented intrusion” into newsgathering activities.

In a letter to Pruitt dated Tuesday, Cole, the deputy attorney general supervising the probe, said the Justice Department seeks news organizations’ phone records only when there are “reasonable grounds to believe that a federal crime has been committed and that the information sought by the subpoena is essential to a successful investigation.” He said such records are subpoenaed “only after all other reasonable alternative investigative steps have been taken,” which in this case included “conducting over 550 interviews and reviewing tens of thousands of documents.”

Cole also said the department “did not seek the content of any calls.”

The media coalition, headed by the Reporters Committee for Freedom of the Press and including major U.S. newspapers and television networks, responded that its members were “stunned” to learn of the Justice Department’s action.

“In the thirty years since the Department issued guidelines governing its subpoena practice as it relates to phone records from journalists, none of us can remember an instance where such an overreaching dragnet for newsgathering materials was deployed by the Department, particularly without notice to the affected reporters or an opportunity to seek judicial review,” the letter said. It charged that Justice “appears to have ignored or brushed aside almost every aspect” of its own guidelines governing subpoenas of the news media for testimony and evidence.

The signatories also called on the department to “explain how government lawyers overreached so egregiously in this matter” and describe what it would do “to mitigate the impact of these actions.” And it said Justice “should announce whether it has served any other pending news media-related subpoenas that have not yet been disclosed.”

The aggressive investigation into the possible disclosure of classified information to the AP is part of a pattern in which the Obama administration has pursued current and former government officials suspected of releasing secret material. Six officials have been prosecuted, more than under all previous administrations combined.

In a letter to Holder, Pruitt said the Justice Department sought information beyond what could be justified by any specific probe. He demanded that the government return the phone records and destroy all copies.

“There can be no possible justification for such an overbroad collection of the telephone communications of The Associated Press and its reporters,” Pruitt wrote to Holder. “These records potentially reveal communications with confidential sources across all of the newsgathering activities undertaken by the AP during a two-month period, provide a road map to AP’s newsgathering operations, and disclose information about AP’s activities and operations that the government has no conceivable right to know.”

The inquiry is one of two leak investigations ordered last June by Holder. The second involves a New York Times report about the Stuxnet computer worm, which was developed jointly by the United States and Israel to damage nuclear centrifuges at Iran’s main uranium-enrichment plant.

The two leak inquiries were started after Republicans in Congress accused the Obama administration of orchestrating news stories intended to demonstrate the president’s toughness on terrorism and improve his chance for reelection. The Republicans sought a special prosecutor, but Holder instead named two veteran prosecutors to handle the inquiries.

In the AP case, the news organization and its reporters and editors are not the likely targets of the investigation. Rather, the inquiry is probably aimed at current or former government officials who divulged classified information.

But experts said the scope of the records secretly seized from the AP and its reporters goes beyond the known scale of previous leak probes.

“This investigation is broader and less focused on an individual source or reporter than any of the others we’ve seen,” said Steven Aftergood, a government secrecy expert at the Federation of American Scientists. “They have swept up an entire collection of press communications. It’s an astonishing assault on core values of our society.”

The office of the U.S. attorney for the District of Columbia on Monday released a statement saying it is not required to notify a media organization in advance of issuing such subpoenas if doing so “would pose a substantial threat to the integrity of the investigation.”

“We take seriously our obligations to follow all applicable laws, federal regulations, and Department of Justice policies when issuing subpoenas for phone records of media organizations,” said a statement from Bill Miller, spokesman for the office. “Those regulations require us to make every reasonable effort to obtain information through alternative means before even considering a subpoena for the phone records of a member of the media.”

Justice Department guidelines require that subpoenas of records from news organizations must be approved personally by the attorney general. Holder’s office did not reply Monday to repeated requests for comment.

Lucy A. Dalglish, dean of the journalism school at the University of Maryland, said she thought the scope of the seizure was unprecedented. “Unfortunately, the Justice Department does this now and again,” said Dalglish, former executive director of the Reporters Committee for Freedom of the Press. “What’s very unusual is the scope of the subpoenas.”

Republicans also quickly condemned the targeting of journalists and sought to portray the Justice Department’s actions as part of a pattern of Obama administration overreach, noting that the Internal Revenue Service was already enmeshed in a scandal over the reported targeting of conservative groups.

“Coming within a week of revelations that the White House lied to the American people about the Benghazi attacks and the IRS targeted conservative Americans for their political beliefs, Americans should take notice that top Obama administration officials increasingly see themselves as above the law and emboldened by the belief that they don’t have to answer to anyone,” said Rep. Darrell Issa (R-Calif.), chairman of the House Oversight and Government Reform Committee.

Senate Judiciary Committee Chairman Patrick J. Leahy (D-Vt.) said in a statement that he is “very troubled by these allegations” and wants to hear the government’s explanation. “The burden is always on the government when they go after private information — especially information regarding the press or its confidential sources,” Leahy said. “I want to know more about this case, but on the face of it, I am concerned that the government may not have met that burden.”

In the AP investigation, Pruitt said, the Justice Department obtained the records without notifying his news organization or narrowing the scope of its subpoenas to specific matters relevant to an ongoing investigation.

The story at issue included details of a CIA operation in Yemen that foiled an al-Qaeda plot in the spring of 2012 to set off a bomb on an airplane headed to the United States. The April and May 2012 phone records of the reporters and editor of the story were among the material seized by the Justice Department.

Pruitt said that on Friday, the AP’s general counsel, Laura Malone, received a letter from Ronald C. Machen Jr., the U.S. attorney for the District of Columbia, who was appointed by Holder to lead the AP inquiry. He said that last year, the Justice Department obtained telephone records for more than 20 separate phone lines assigned to the AP and its journalists.

A grand jury based in Washington’s federal court has been investigating the possible leak of classified information to the AP for several months, according to a government official familiar with the probe who spoke on the condition of anonymity to discuss an ongoing case.

In most cases when investigators seek information about a media organization’s source, the news organization moves to quash any subpoena for its records. But in this case, the AP was not aware that the records had been obtained.

Law enforcement officials say leak cases are difficult because hundreds of people often have access to the classified information. In the AP case, knowledge of the details of the al-Qaeda plot would have been highly compartmentalized, which would limit the number of people with access to it. A former government investigator said telephone records could be valuable in narrowing the pool of suspected leakers.

Numerous senior government officials have been interviewed in connection with the investigation into the AP story. Among those questioned was John O. Brennan, who served as Obama’s counterterrorism adviser before becoming CIA director this year.

In his confirmation hearing in February, Brennan said he had cooperated voluntarily with the Justice Department and had been interviewed. He acknowledged speaking to former U.S. government officials who work as commentators on television news, but he denied sharing any classified information.

Greg Miller, Carol D. Leonnig and Julie Tate contributed to this report.