(CNN) -- The spiraling scandal that took down David Petraeus has apparently ensnared another powerful general, as authorities announced that Gen. John Allen is under investigation for allegedly sending inappropriate messages to Jill Kelley, a woman who has been linked to the Petraeus scandal.
Allen, who is the commander of NATO's International Security Assistance Force, has denied any wrongdoing, a senior defense official said.
Details of the latest angle of the scandal that has shaken the highest level of the military were sketchy early Tuesday morning.
Some details about Allen, the top U.S. commander in Afghanistan, came from a terse overnight statement by Defense Secretary Leon Panetta.
"On Sunday, the Federal Bureau of Investigation referred to the Department of Defense a matter involving General John Allen, Commander of the International Security Assistance Force (or ISAF) in Afghanistan," part of the statement said. "Today, the secretary directed that the matter be referred to the Inspector General of the Department of Defense for investigation."
A defense official told CNN there is a "distinct possibility" that the investigation into Allen is connected to the investigation that led to the resignation of Petraeus.
Allen will still retain his position as the commander of ISAF as the investigation continues, the Pentagon said.
But Panetta asked that Allen's nomination to become NATO's supreme allied commander be put on hold, the statement said.
The confirmation hearing to see if Allen would get that lofty military post was scheduled for Thursday.
The investigation was in its early stages but authorities were looking into some 20,000 to 30,000 pages of documents, the defense official said.
It was not immediately clear Tuesday exactly how Allen may be linked to the Petraeus investigation.
Petraeus, 60, resigned Friday after acknowledging he had an affair with a woman later identified as his biographer, Paula Broadwell, 40, a fellow West Point graduate who spent months studying the general's leadership of U.S. forces in Afghanistan.
The investigation into Petraeus came to light during an FBI investigation of "jealous" e-mails reportedly sent by Broadwell to a woman named Jill Kelley, a government source familiar with the investigation told CNN on Monday.
Now authorities say Allen was under investigation because of communications with Kelley.
Amid national talk about the Petraeus scandal, Kelley, 37, and her husband released a statement saying they have been friends with Petraeus and his family for more than five years and asked for privacy.