WASHINGTON (AP) — The sex scandal that led to CIA Director David Petraeus' resignation widened today, 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. Some of the 20 000-plus pages of documents and emails between Gen. John Allen and Florida socialite Jill Kelley were "flirtatious," a senior defense official told The Associated Press. The defense official spoke on condition of anonymity because he wasn't authorized to discuss the case publicly. Defense Secretary Leon Panetta revealed earlier today 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, National Security spokesman Tommy Vietor said President Barack Obama "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. 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. Broadwell's threatening emails to Kelley, a 37-year-old Petraeus family friend, led to the FBI's discovery of communications between Broadwell and Petraeus indicating they were having an affair.