According to Wired, the letter sent by the office of the U.S. Attorney Ronald Machen, Jr. contained no explanation about why the department needed those records.
It simply stated the phone numbers in question, and contained a citation of a section in the Justice Department guidelines that explained that the federal authorities were allowed to delay the sending of the notice until it no longer poses "a clear and substantial threat to the integrity of the investigation."
The same letter was also sent to the counsel by email, and as a PDF file to four reporters and an editor, but not to the other journalists. This was taken as a confirmation of the speculation that the DOJ was interested ferreting out which federal officials were leaking confidential information to the press, as the journalists in questions worked on a story about a 2012 al-Qaida terror plot thwarted by the CIA.
According to the AP, the government gathered records of more than 20 phone lines, including the home lines of some of the reporters and editors and the official phone numbers of the organization's offices in New York, Washington and Hartford, as well as the number used by its press corps in the press gallery of the House of Representatives. The AP pointed out that most of those phone lines are also used by more than 100 of its journalists on a daily base.
It seems that the records obtained by the DOJ were only for outgoing calls, but it's still not confirmed.
In any case, AP CEO Gary Pruitt protested to Attorney General Eric Holder via a letter, saying that "there can be no possible justification for such an overbroad collection of the telephone communications of The Associated Press and its reporters. 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," and asked for the records to be destroyed.
DOJ's move was questioned by a number of politicians, and was condemned by the American Civil Liberties Union.
"Obtaining a broad range of telephone records in order to ferret out a government leaker is an unacceptable abuse of power. Freedom of the press is a pillar of our democracy, and that freedom often depends on confidential communications between reporters and their sources," said Ben Wizner, director of the ACLU Speech, Privacy, and Technology Project.
"The media's purpose is to keep the public informed and it should be free to do so without the threat of unwarranted surveillance. The Attorney General must explain the Justice Department's actions to the public so that we can make sure this kind of press intimidation does not happen again," demanded Laura Murphy, director of the American Civil Liberties Union Washington Legislative Office.