PERTH, Australia -- Gen. John Allen, the top American and NATO commander in Afghanistan, is under investigation for what a senior defense official said early Tuesday was "inappropriate communication'' with Jill Kelley, the woman in Tampa who was seen as a rival for David H. Petraeus's attentions by Paula Broadwell, the woman who had an extramarital affair with Mr. Petraeus.
In a statement released to reporters on his plane en route to Australia early Tuesday, Defense Secretary Leon E. Panetta said that the F.B.I. had informed him on Sunday of its investigation of General Allen.
Mr. Panetta turned the matter over to the Pentagon's inspector general to conduct its own investigation into what the defense official said were 20,000 to 30,000 pages of documents, many of them e-mails between General Allen and Ms. Kelley, who is married with children.
Asked if the F.B.I. had determined that there was criminal action involved, the defense official replied, "That is for the F.B.I. to discuss.'' The official, who briefed reporters on Mr. Panetta's plane, said that "there is the distinct possibility'' that the e-mails were connected to an ongoing F.B.I. investigation into Mr. Petraeus and Ms. Broadwell.
The defense official said that General Allen, who is also married, told Pentagon officials he had done nothing wrong. Neither he nor Ms. Kelley could be reached for comment early Tuesday. Mr. Panetta's statement praised General Allen for his leadership in Afghanistan and said that "he is entitled to due process in this matter.''
But the two investigations open up what could be widening scandal into two of the most prominent generals of their generation – Mr. Petraeus, who was the top commander in Iraq and Afghanistan before he retired from the military and became director of the C.I.A., only to resign on Friday because of the affair, and General Allen, who also served in Iraq and now commands 68,000 American troops in Afghanistan.
Although General Allen will remain the commander in Afghanistan, Mr. Panetta said that he had asked President Obama to put on hold General Allen's nomination to be the commander of American forces in Europe and the Supreme Allied Commander of NATO, two positions he was to move into after what was expected to be easy confirmation by the Senate. Mr. Panetta said in his statement that Mr. Obama agreed with his decision.
Gen. Joseph A. Dunford, the assistant commandant of the Marine Corps who was nominated last month by Mr. Obama to succeed General Allen in Afghanistan, will proceed as planned with his confirmation hearing. In his statement, Mr. Panetta urged the Senate to act promptly on his nomination.
The defense official said that the e-mails between Ms. Kelley and General Allen spanned the years 2010 to 2012. The official could not explain why there were so many pages of e-mails and did not specify their content. The official said he could not explain how the e-mails between Ms. Kelley and General Allen were related to the e-mails between Mr. Petraeus and Ms. Broadwell and e-mails between Ms. Broadwell and Ms. Kelley.
In what is known so far, Ms. Kelley went to the F.B.I. last summer after she was disturbed by harassing e-mails. The F.B.I. began an investigation and learned that the e-mails were from Ms. Broadwell. In the course of looking into Ms. Broadwell's e-mails, the F.B.I. discovered e-mails between Ms. Broadwell and Mr. Petraeus that indicated they were having an extramarital affair. Ms. Broadwell, officials say, saw Ms. Kelley as a rival for her affections with Mr. Petraeus.
The defense official said he did not know how General Allen and Ms. Kelley knew each other. General Allen has been in Afghanistan as the top American commander since July 2011, although before that he lived in Tampa as the deputy commander for Central Command, which oversees American military operations in the Middle East.
The defense official said that the Pentagon had received the 20,000 to 30,000 pages of documents from the F.B.I. and was currently reviewing them.
The defense official said that at 5 p.m. Washington time on Sunday Mr. Panetta was informed by the Pentagon's general counsel of the F.B.I. investigation into General Kelley. Mr. Panetta was at the time on his plane en route from San Francisco to Honolulu, his first stop on a weeklong trip to the Pacific and Asia. Mr. Panetta notified the White House and then the leaders of the Senate and House Armed Services Committee.
General Allen is now in Washington for what was to be his confirmation hearing as commander in Europe. That hearing, the official said, will now be delayed.
This article originally appeared in The New York Times.