WASHINGTON - The FBI probe into the sex scandal that prompted CIA Director David Petraeus to resign has expanded to ensnare Gen. John R. Allen, the commander of U.S. and NATO troops in Afghanistan, the Pentagon announced Tuesday. The same investigation is seeking to determine how Paula Broadwell, the woman who had an affair with Petraeus, obtained classified files.

According to a senior U.S. defense official, the FBI has uncovered between 20,000 and 30,000 pages of documents -- most of them e-mails -- that contain "potentially inappropriate" communication between Allen and Jill Kelley, a Florida woman whose report of harassment by a person who turned out to be Broadwell ultimately led to Petraeus' downfall.

Meanwhile, senior law enforcement officials said that a late-night seizure on Monday of boxes of material from the North Carolina home of Broadwell 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, his biographer, have denied to investigators that he was the source of any classified information, officials said.

The surprise move by the FBI follows previous 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.

Key lawmakers signaled Tuesday their intent to scrutinize Justice's handling of an inquiry that focused initially on a potential conflict between two private people but quickly morphed into an exhaustive 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."

Allen, a Marine, succeeded Petraeus as the top allied commander in Afghanistan in July 2011. He also served as Petraeus' deputy when both generals led the military's Tampa-based Central Command from 2008 until 2010. He had been nominated by the White House to take over as chief of the military's European Command and NATO's Supreme Allied Commander in Europe, but that nomination is on hold.

A senior U.S. official who is close to Allen strongly denied that the general and Kelley had an affair or engaged in inappropriate communication. Allen and Kelley, who threw parties and other social events involving senior leaders at the Central Command, did exchange "a few hundred e-mails over a couple of years." "He's never been alone with her," the senior official said. "Did he have an affair? No."

A Senate aide said that the Pentagon provided limited information on the nature of the Allen e-mails, but that the decision to investigate suggests that defense officials see no conclusive evidence of wrongdoing.

"Obviously, they think that the contents of all these e-mails are at the least ambiguous," the aide said. "But if they thought that it was clear and pointed to evidence of an affair, I assume they would withdraw his nomination and relieve him."

Adulterous relationships are outlawed under military code.

The senior official said Allen received at least one e-mail about Kelley from an unidentified account that eventually was traced to Broadwell.

Allen discussed that e-mail with Kelley, who was receiving a slew of harassing messages at the time, the senior official who is close to Allen said. Kelley, who did not know the messages were being sent by Broadwell, notified the FBI.

Some of the e-mails may have prompted suspicions among FBI investigators because Allen sometimes used words such as "sweetheart" to refer to her, the senior official said. But the official added that Allen, who was raised in Virginia, employed that language as a term of platonic friendship, not romantic interest.

Law enforcement officials offered conflicting accounts of the significance of the FBI's seizure of materials from Broadwell's home. Officials said Broadwell provided access to the residence, which the FBI first requested on Sunday, and turned over computers, files and other material. She was not present when the search occurred over several hours Monday night.

The unfolding scandal has shaken Obama's national-security staff and upended his plans for filling military and intelligence leadership jobs in his second term.

Petraeus' fall from grace shocked the CIA but especially stunned his former colleagues in the Army, where he was considered one of the most brilliant and influential commanders of his generation. Allen, a Marine, is likewise seen as an intellectual and upstanding role model who first made his mark as a general in Iraq during the George W. Bush administration and later earned Obama's confidence.

Pentagon chief Leon Panetta said Allen would remain as commander of U.S. and NATO forces in Afghanistan for now, "while the matter is under investigation and before the facts are determined."

But his time as commander in Afghanistan may be short. Panetta has also asked the Senate to expedite the confirmation of Allen's likely successor, Marine Gen. Joseph F. Dunford.

Obama nominated Dunford last month to replace Allen. Coincidentally, the Senate Armed Services Committee had already scheduled his confirmation hearing for Thursday.

The Obama administration is in the final stages of determining its timeline for withdrawing the remaining 68,000 U.S. troops in Afghanistan and is also debating how many trainers, Special Operations Forces and other military assets it will keep in the country after 2014, when the White House has pledged to end combat operations and hand over security to the Afghan national army and police.

On Monday, Panetta told reporters that Allen had recently presented options to the Pentagon and White House for what the U.S. military presence could look like in Afghanistan in 2015 and beyond.