Gen. Allen, who had been investigated after emailing a Tampa socialite involved in the David Petraeus scandal, had not engaged in inappropriate behavior according to the Department of Defense Inspector General. NBC's Brian Williams reports.

The Defense Department’s inspector general has found that allegations that Gen. John Allen engaged in inappropriate behavior in emails he exchanged with Tampa, Fla., socialite Jill Kelley were unsubstantiated.

"The IG cleared him," a defense official told NBC News, speaking on condition of anonymity.

Allen, the current commander of NATO forces in Afghanistan, has been nominated to be Supreme Allied Commander in Europe, but that promotion was put on hold when the IG investigation began. The defense official stressed that no decision has been made yet on whether his nomination will go forward again.

A statement Wednesday by the International Security Assistance Force in Afghanistan on behalf of Allen said he had been informed that investigators “the allegations against him were unsubstantiated and … that he did not violate the requirement of exemplary conduct or the prohibition against conduct unbecoming an officer and a gentleman. “

“He's obviously pleased by the outcome,” it said. “But more critically, he is grateful for the support he received throughout this process from his chain of command, friends, family and colleagues. He remains focused, as he has always been, on leading the brave men and women of the ISAF team."

Allen’s nomination was jeopardized in mid-November when it was revealed that he had exchanged emails with Kelley that some Pentagon officials at the time characterized as “inappropriate” and “flirtatious.”

Allen had met Kelley when he was commander of MacDill Air Force Base outside Tampa, where she served as a volunteer “social liaison.”

She inadvertently drew him into the scandal that led to the resignation of former CIA Director David Petraeus by complaining to an FBI agent with whom she was acquainted about anonymous emails referencing Petraeus. Among those emails was one that Allen had forwarded to her in the belief that she had sent it to him as a joke, officials told NBC News at the time.

FBI agents eventually traced the allegedly threatening emails to Paula Broadwell, Petraeus' biographer. That investigation also led to evidence of an extramarital affair between Petraeus and Broadwell, prompting his resignation on Nov. 7. Days later, Defense Secretary Leon Panetta announced the investigation into Allen’s relationship with Kelley.

Jill Kelley speaks out: 'I knew I was being stalked'

The inspector general’s investigation does not determine guilt or innocence. Rather, it decides whether an allegation is substantiated or not. Even if the finding is that the allegation is unsubstantiated, the IG can still make a recommendation that can harm an officer’s career.

Jim Miklaszewski is NBC News' Chief Pentagon Correspondent; Courtney Kube is an NBC News producer at the Pentagon. Mike Brunker, NBC News investigations editor, contributed to this report.

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