(CNN) -- The Justice Department secretly collected two months of telephone records for reporters and editors at The Associated Press, the news service disclosed Monday in an outraged letter to Attorney General Eric Holder.
The records included calls from several AP bureaus and the personal phone lines of several staffers, AP President Gary Pruitt wrote. Pruitt called the subpoenas a "massive and unprecedented intrusion" into its reporting.
"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," wrote Pruitt, the news agency's CEO.
The AP reported that the government has not said why it wanted the records. But it noted that U.S. officials have said they were probing how details of a foiled bomb plot that targeted a U.S.-bound aircraft leaked in May 2012. The news agency said records from five reporters and an editor who worked on a story about the plot were among those collected, but it said none of the information the government has shared with it suggested agents listened in on any reporters' calls.
The news immediately raised questions among members of Congress.
"The First Amendment is first for a reason," said Michael Steel, a spokesman for House Speaker John Boehner. "If the Obama administration is going after reporters' phone records, they better have a damned good explanation."
The subpoenas were disclosed to the news agency on Friday, Pruitt wrote. In all, federal agents collected records from more than 20 lines, including personal phones and AP phone numbers in New York; Hartford, Connecticut; and Washington, he wrote.
"We regard this action by the Department of Justice as a serious interference with AP's constitutional rights to gather and report the news," he told Holder. Pruitt demanded that the department return all records collected and destroy all copies.
The U.S. attorney's office in Washington responded that federal investigators seek phone records from news outlets only after making "every reasonable effort to obtain information through alternative means." It did not disclose the subject of the probe.
"We must notify the media organization in advance unless doing so would pose a substantial threat to the integrity of the investigation," it said. "Because we value the freedom of the press, we are always careful and deliberative in seeking to strike the right balance between the public interest in the free flow of information and the public interest in the fair and effective administration of our criminal laws."
CNN legal analyst Jeffrey Toobin said the Obama administration "has been incredibly aggressive" about prosecuting leakers, and there's no privilege in federal law that allows reporters to protect their sources. But he said past administrations have avoided going that far.
"I have never heard of a subpoena this broad," Toobin said. "It's legal, as far as I can tell. The administration isn't violating the First Amendment. But they are certainly doing more than has ever been done before in pursuing the private information of journalists. And we'll see if there's any political check on them, because there doesn't appear to be any legal check on what they're doing."
The White House was unaware of the subpoenas, spokesman Jay Carney told reporters Monday night.
"We are not involved in decisions made in connection with criminal investigations, as those matters are handled independently by the Justice Department," Carney said.
Holder announced in June 2012 that he had assigned two U.S. attorneys to lead investigations into the possible leaking of state secrets, and members of Congress have complained about disclosures of electronic warfare campaigns against Iran, U.S. drone attacks overseas and Obama's personal involvement in "kill lists" of militants in Yemen and Pakistan.
But Pruitt wrote that most of the records collected from the AP "can have no plausible connection to any ongoing investigation," and the American Civil Liberties Union called on the Justice Department to explain its actions.
"Obtaining a broad range of telephone records in order to ferret out a government leaker is an unacceptable abuse of power," Ben Wizner, the head of the ACLU's Speech, Privacy, and Technology Project, said in a written statement. "Freedom of the press is a pillar of our democracy, and that freedom often depends on confidential communications between reporters and their sources."
In a statement issued Monday night, Senate Judiciary Committee Chairman Patrick Leahy said, "I am very troubled by these allegations and want 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," said Leahy, D-Vermont. "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."
And Rep. Darrell Issa, R-California, told CNN that the Justice Department already has the ability "to listen, very transparently, to all the government phones and government activities."
"You can imagine if Congress wanted to know about leaks that obviously came out of the administration that ended up in the press, they would be outraged if we tried to get that information," said Issa, a member of the House Judiciary Committee and a leading critic of Holder. "But that's exactly what they're doing. They're looking at what is considered to be confidential."