Allen succeeded Petraeus as the top American commander in Afghanistan in July 2011, and his nomination to become the next commander of U.S. European Command and the commander of NATO forces in Europe has now 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, Petraeus' biographer, with whom he had an affair that led to his abrupt resignation Friday. It was Broadwell's threatening emails to Jill Kelley, a Florida woman 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 said 20,000 to 30,000 pages of emails and other documents from Allen's communications with Kelley between 2010 and 2012 are under review. He would not say whether they involved sexual matters or 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 any wrongdoing.
The new details in the case brought expressions of amazement in Congress, already in an uproar over the probe that has roiled the intelligence and military establishment.
Rep. Peter King, R-N.Y., chairman of the House Homeland Security Committee, called the case "a Greek tragedy.
"It's just tragic," King said on NBC's "Today" show. "This has the elements in some ways of a Hollywood movie or a trashy novel."