The author of the Rolling Stone article that ended the military career of Gen. Stanley McChrystal, the former top commander in Afghanistan, has been denied permission to join U.S. troops fighting in Afghanistan, the Pentagon said Tuesday.

Defense Department spokesman Col. David Lapan told reporters that freelance writer Michael Hastings was rebuffed when he asked to accompany, or "embed," with American forces next month.

The rejection came as the Pentagon ramped up an internal investigation into the circumstances behind some of the most salacious material Hastings used in his article in Rolling Stone. The Army inspector general is interviewing current and former McChrystal aides, The Associated Press has learned.

The inspector general's review began shortly after Rolling Stone published the article that torpedoed McChrystal's three-decade Army career.

The inspector general, an independent investigator, is considering whether officers were insubordinate and how far up the chain of command responsibility for decisions involving the Hastings interviews extended, officials said. Defense officials outlined the investigation on condition of anonymity because it is ongoing and has reached no conclusions.

Hastings quoted McChrystal and his aides criticizing and mocking Obama administration officials, including Vice President Joe Biden. McChrystal was recalled to Washington and fired.

Lapan acknowledged that it's "fairly rare" for the military to turn way a reporter who wants to embed with front-line troops.

"There is no right to embed," Lapan said. "It is a choice made between units and individual reporters, and a key element of an embed is having trust that the individuals are going to abide by the ground rules. So in that instance the command in Afghanistan decided there wasn't the trust requisite and denied this request."

Lapan did not say what unit Hastings had asked to accompany or whether he had spelled out his assignment. He is a freelance reporter currently working on a story about helicopters in Afghanistan, but also has signed a book contract that grew out of the McChrystal story.

Hastings did not immediately reply to requests for comment Tuesday. He has said he did nothing wrong in chronicling the banter, profanity and jocular insults among McChrystal's inner circle. In Twitter tweet late Tuesday, Hastings said he refused to participate in the army's IG investigation. Later, he tweeted: "the embed had already been approved for september. now it has been disapproved."

The four-star general retired in a Pentagon ceremony July 23.

In his 18-minute farewell to arms, delivered to a crowd of VIPs, McChrystal made light of the episode. He warned his comrades in arms: "I have stories on all of you, photos of many, and I know a Rolling Stone reporter."