The top U.S. commander in Afghanistan, General John Allen, is under investigation for allegedly inappropriate communication with a woman at the centre of the scandal involving former CIA Director David Petraeus.
The shocking revelation threatens to fell another one of the U.S. military's biggest names and suggests that the scandal involving Petraeus - a former four-star general who had Allen's job in Afghanistan before moving to the CIA last year - could expand much further than previously imagined.
Speaking on condition of anonymity, a senior U.S defence official said the FBI uncovered between 20,000 and 30,000 pages of communications - mostly emails starting in 2010 - between Allen and Jill Kelley, who has been identified as a long-time friend of the Petraeus family and a Tampa, Florida, volunteer social liaison with military families at MacDill Air Force Base.
It was Kelley's complaints about harassing emails from the woman with whom Petraeus had had an affair, Paula Broadwell, that prompted an FBI investigation, ultimately alerting authorities to Petraeus' involvement with Broadwell.
Petraeus resigned from his job on Friday.
Asked whether there were concerns about the disclosure of classified information, the official said: "We are concerned about inappropriate communications. We are not going to speculate as to what is contained in these documents."
Defence Secretary Leon Panetta said in a statement given to reporters flying with him to Australia that he asked that Allen's nomination to be Commander of U.S. European Command and Supreme Allied Commander Europe be delayed "and the president has agreed."
Allen, who is now in Washington, was due to face a Senate confirmation hearing on Thursday (local time), as was his slated successor in Afghanistan, General Joseph Dunford.
The FBI referred the case to the Pentagon on Sunday and Panetta directed the Defence Department's Inspector General to handle the investigation.
Panetta informed the top Republican and Democrat on the Senate Armed Services Committee during the flight to Australia. The House Armed Services Committee was also notified.
The U.S. defence official said that Allen denied any wrongdoing and that Panetta had opted to keep him in his job while the matter was under review, and until Dunford can be confirmed to replace him - a process that gains urgency given the potentially lengthy review process and the cloud it could cast over the mission in Afghanistan.
"While the matter is under investigation and before the facts are determined, General Allen will remain commander of ISAF," Panetta said, referring to the NATO-led force in Afghanistan.
Only hours earlier, Panetta had said he was reviewing Allen's recommendations on the future U.S. presence in Afghanistan after most troops withdraw by the end of 2014.
Commending Allen's leadership in Afghanistan, Panetta said in his statement: "He is entitled to due process in this matter."
At the same time, he noted that wanted the Senate to act "promptly" on Dunford's nomination.
The U.S. official said Panetta was informed of the matter involving Allen on Sunday, as he flew to Hawaii, after the Pentagon's top lawyer called Panetta's chief of staff. The White House was informed next.