WASHINGTON (AP) -- The sex scandal that led to CIA Director David Petraeus' resignation widened Tuesday, with the top U.S. commander in Afghanistan under investigation for alleged "inappropriate communications" with a woman who received threatening emails from Petraeus' former lover.
A senior defense official told The Associated Press that some of the 20,000-plus pages of documents and emails between Gen. John Allen and Florida socialite Jill Kelley were "flirtatious."
It wasn't immediately clear whether Allen was flirting with Kelley or whether he was the recipient of flirtatious emails. The defense official spoke on condition of anonymity because he wasn't authorized to discuss the case publicly.
Defense Secretary Leon Panetta revealed earlier Tuesday that the Pentagon had begun an internal investigation into the emails.
Allen succeeded Petraeus as the top American commander in Afghanistan in July 2011, and his nomination to become the next commander of the U.S. European Command and the commander of NATO forces in Europe has been put on hold.
In a White House statement early Tuesday, National Security spokesman Tommy Vietor said President Barack Obama has held Allen's nomination at Panetta's request. Obama, the statement said, "remains focused on fully supporting our extraordinary troops and coalition partners in Afghanistan, who Gen. Allen continues to lead as he has so ably done for over a year."
Meanwhile, FBI agents searched the home of Paula Broadwell, the 40-year-old biographer with whom Petraeus had an affair that led to his abrupt resignation Friday.
The resignation and his acknowledgement of an affair stunned Washington because of the former general's highly disciplined and well-praised career, which had led some to speculate whether a run for the White House would be in his future. The 60-year-old retired Army general headed U.S. military operations in Iraq and Afghanistan before taking charge of the CIA.
It was Broadwell's threatening emails to Kelley, a 37-year-old who is a Petraeus family friend, that led to the FBI's discovery of communications between Broadwell and Petraeus indicating they were having an affair.
A Pentagon official earlier Tuesday said 20,000 to 30,000 pages of emails and other documents from Allen's communications with Kelley between 2010 and 2012 were under review. He would not say whether they are thought to include unauthorized disclosures of classified information. He said he did not know whether Petraeus is mentioned in the emails.
Allen has denied wrongdoing. If Allen was found to have had an affair with Kelley, he could face charges of adultery, which is a crime under the Uniform Code of Military Justice.
The new details in the case brought expressions of amazement in Congress, already in an uproar over the Petraeus affair.
Rep. Peter King, chairman of the House Homeland Security Committee, called the case "a Greek tragedy.
"It's just tragic," King said on NBC. "This has the elements in some ways of a Hollywood movie or a trashy novel."
Lawmakers have said they should have been told earlier about the affair and are asking what the FBI knew and when it notified top Obama administration officials.
The White House wasn't informed of the FBI investigation that involved Petraeus until Nov. 6, Election Day, although agents began looking at Petraeus' actions months earlier, sometime during the summer. Senate Intelligence Committee chairwoman Dianne Feinstein said she first learned of the matter from the media late last week and confirmed it in a phone call to the then-CIA director on Friday.
Obama on Friday accepted Petraeus' resignation.
The Justice Department -- of which the FBI is part -- is supposed to refrain from sharing detailed information about its criminal investigations with the White House.
The FBI also looked into whether a separate set of emails between Petraeus and Broadwell might involve any security breach. That will be a key question Wednesday in meetings involving congressional intelligence committee leaders, FBI deputy director Sean Joyce and CIA deputy director Michael Morell.
A federal law enforcement official, speaking on condition of anonymity to discuss details of the investigation, said the FBI had concluded relatively quickly -- and by late summer at the latest -- that there was no security breach. Absent a security breach, it was appropriate not to notify Congress or the White House earlier, this official said.
Extramarital affairs are viewed as particularly risky for intelligence officers, though it is not a crime, because they might be blackmailed to keep the affair quiet.
According to two federal law enforcement officials, the FBI initially began a criminal investigation of unsigned, harassing emails that were sent, beginning last May, to Kelley. She and her husband, Scott, were longtime friends of Petraeus and his wife, Holly. FBI agents traced the alleged cyber harassment to Broadwell and discovered she was exchanging intimate