The Justice Department used a secret subpoena to obtain two months of phone records for Associated Press reporters and editors without notifying the news organization, a senior department official tells NBC News, saying the step was necessary to avoid “a substantial threat to the integrity” of an ongoing leak investigation.

The seizure of the phone records, disclosed earlier Monday by AP President and CEO Gary Pruitt, is the latest move in a series of high profile and controversial investigations of leaks of classified information by the Justice Department. In a letter of protest to Attorney General Eric Holder, Pruitt called said obtaining more than two months of AP phone records on 20 separate telephone lines without prior notice was a “massive and unprecedented intrusion” into news-gathering operations.

It also drew a swift rebuke Monday from members of Congress and freedom of the press watchdogs, one of whom called the move “Nixonian.”

Ronald C. Machen, Jr., the U.S. attorney in Washington, D.C., revealed in a letter to the AP on Friday that federal prosecutors obtained the records. The letter did not give a reason for obtaining the records, but Machen is conducting an investigation into the leak of classified information about a foiled terror plot in Yemen last year. An AP story last spring reported details of a CIA operation in Yemen that stopped an al Qaeda plot to detonate a bomb on an airplane bound for the United States.

More: AP Calls Government’s Record Seizure a ‘Massive and Unprecedented Intrusion’ - Open Channel