WASHINGTON -- The sex scandal that felled CIA Director David Petraeus widened Tuesday to possibly taint the top U.S. commander in Afghanistan, Marine Gen. John Allen, in a suddenly public drama involving a Tampa socialite, a jealous rival, a twin sister in a messy custody dispute and flirty emails.

The improbable story — by turns tragic and silly — could have major consequences, unfolding at a critical time in the Afghan war effort and just as President Barack Obama was hoping for a smooth transition in his national security team.

The unexpected turn of events prompted Obama to put on hold his nomination of Allen to be the top NATO commander in Europe, pending the outcome of a Pentagon investigation into “inappropriate” emails that U.S. defense officials said Allen had sent to a central figure in the scandal, which was ignited by Petraeus’ admission to an extramarital affair.

The White House said that Obama retained his faith in Allen, who is in the midst of developing a plan for the gradual withdrawal of U.S.-led international forces from Afghanistan by the end of 2014.

“The president remains focused on fully supporting our extraordinary troops and coalition partners in Afghanistan who General Allen continues to lead as he has done so ably for over a year,” said White House Press Secretary Jay Carney, adding that the president “believes he’s doing and has done an excellent job.”

As Congress returned from its election break Tuesday, some lawmakers expressed dismay at reports that Allen was able to exchange more than 20,000 documents and emails with Tampa socialite Jill Kelley, even as he oversaw the longest war in the nation’s history.

“It’s hard to imagine you can manage an agency or run a theater of war while sending or getting so many emails, if that’s what actually occurred,” Sen. Richard Durbin of Illinois, the Senate’s No. 2 Democrat, told McClatchy News Service.

Senate Armed Services Committee Chairman Carl Levin, D-Mich., who agreed to Obama’s request to delay Allen’s nomination hearing, which had been set for Thursday, also said he was “kind of surprised by the number of emails, but until the … investigation concludes, I don’t want to prejudge it.”

The committee, however, will proceed with the confirmation hearing of Marine Gen. Joseph F. Dunford to take over from Allen as commander in Afghanistan. The handover is not expected before March.

In a related development, Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., said that she intended to summon Petraeus back into the spotlight to answer questions about the Sept. 11 attack on the U.S. consulate in the eastern Libyan city of Benghazi. It resulted in the deaths of the U.S. ambassador to Libya, two CIA contract security guards and a State Department staffer. The committee hasn’t decided when he would be called, she told McClatchy News Service.

“Of course I’m dismayed,” Feinstein said. “It’s a difficult situation. We’re trying to be responsible.”

Carney said that it was “up to Congress to make decisions about who is called to testify.”

Feinstein and other lawmakers have expressed concern that they weren’t informed about the FBI investigation that uncovered Petraeus’ affair with his biographer, Paula Broadwell, late this summer until after it became public Friday. The CIA released a Petraeus resignation statement to the agency’s workforce in which the storied, retired four-star Army general admitted to committing adultery.