The FBI is making a new push to determine how a woman who had an affair with retired Gen. David H. Petraeus when he was CIA director obtained classified files, part of an expanding series of investigations in a scandal that also threatens the career of the United States’ top military commander in Afghanistan.

Senior law enforcement officials said that a late-night seizure on Monday of boxes of material from the North Carolina home of Paula Broadwell, a Petraeus biographer whose affair with him led to his resignation last week, marks a renewed focus by investigators on sensitive material found in her possession.

“The issue of national security is still on the table,” one U.S. law enforcement official said. Both Petraeus and Broadwell have denied to investigators that he was the source of any classified information, officials said.

The surprise move by the FBI follows assertions by U.S. officials that the investigation had turned up no evidence of a security breach — a factor that was cited as a reason the Justice Department did not notify the White House before last week that the CIA director had been ensnared in an e-mail inquiry.

The disclosure about the FBI’s renewed focus comes as investigations of the matter expanded on other fronts.

The Defense Department said Tuesday that its inspector general is examining hundreds of e-mails between Marine Gen. John R. Allen, the U.S. commander in Afghanistan, and a Florida woman also linked to the Petraeus inquiry.

At the same time, key lawmakers signaled their intent to scrutinize the Justice Department’s handling of an inquiry that focused initially on a potential conflict between two private people but quickly morphed into an examination of the e-mail of two top national security officers.

“My immediate gut is like this is the National Enquirer,” Sen. Dianne Feinstein (D-Calif.), the chairman of the Senate intelligence committee, said in an interview on CNN. “I mean, every day there is something new.”

Feinstein added that she has “many questions about the nature of the FBI investigation, how it was instituted, and we’ll be asking those.”

White House spokesman Jay Carney said that President Obama maintains confidence in Allen and that the four-star Marine Corps general will continue to lead the war in Afghanistan even as he faces the inspector general’s inquiry.

“I can tell you that the president thinks very highly of General Allen and his service to his country, as well as the job he has done in Afghanistan,” Carney said, adding that Obama “has faith in General Allen, believes he’s doing and has done an excellent job.”

At the same time, Carney said Obama put on hold Allen’s nomination to serve as supreme allied commander for NATO forces in Europe, canceling Allen’s appearance before the Senate Armed Services Committee this week.

The Allen investigation focuses on his extensive correspondence with Jill Kelley, a Tampa resident who had carved out a role as an ad-hoc social ambassador to military personnel at MacDill Air Force Base.