The search for who leaked the information is being led by the U.S. Attorney's Office in Washington and has involved extensive FBI interviews of personnel at the Justice Department, U.S. intelligence agencies, the White House's National Security staff and the FBI itself.
The interviews have been lengthy and thorough, said people who have been questioned in the investigation, but requested anonymity. Two of those interviewed said leak inquiries were always aggressive and that being questioned is a wearing and unpleasant experience.
The investigation, which a law enforcement official has said was prompted by a May 7, 2012, AP story about the operation to foil the Yemen plot, appears to be ongoing. Some potential witnesses have been advised they are likely to be interviewed in the next two or three weeks.
Attorney General Eric Holder, who recused himself from involvement in the case, largely sidestepped questions from angry lawmakers on Wednesday about his department's secret seizure of AP records, which the news agency revealed on Monday.