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Retired U.S. Army Lt. Col. John Nagl, one of David Petreaus' friends and fellow servicemen, said the former general told him that his affair with biographer Paula Broadwell did not begin until after he became CIA director.

"The time to come fully clean is now, he understands that and so I believe him," Nagl told ABC's Martha Raddatz. "He covered up his affair for some months; less than a year was the full course of this affair.

"Living conditions in Afghanistan are very close, sparse," he continued. "It is perhaps conceivable, though it would have been very difficult to conduct an affair under those circumstances in that environment."

Watch ABC's Martha Raddatz's latest report on the Petreaus sex scandal here:

Although Nagl said he knew Broadwell, he never saw her and Petreaus together because his trips to Afghanistan didn't overlap with hers. However, Nagl said friends who were with the two of them in Afghanistan told him they were "worried" and "concerned" about Broadwell's "extraordinary access."

"She was a little too close, a little too friendly, she spoke too openly, in my eyes about her access, and she became a little bit too much of his voice," Nagl said. "All of us that worked with him admire him, and I think that's pretty universal. But none of us, other than Paula, would presume to speak for him that way."

A veteran of the Iraq War, Nagl said he has known Petraeus for 25 years. Not only was the former general Nagl's history professor at West Point, the two also co-authored the book, "The U.S. Army/Marine Corps Counterinsurgency Field Manual," together. He was devastated by the news of his mentor's affair.

"I'm really upset and hurt," he said. "This is tarnishing the legacy of one of great American heroes of the last decade."

Nagl said he had not spoken to Petraeus in person, but that the former CIA director apologized to him over email.

"He said that he felt that he had let the team down," Nagl said. "He obviously wanted to make it and pull his family together and try to recover from this, but felt an enormous sense of guilt and regret."

Despite Petreaus' indiscretions, Nagl still supports him, but said he had doubts that the man he still admires will be able to fully come back from the scandal.

"I hope that we haven't seen the end of him in government service, but I'm afraid that may be true," he said. "What I do think that we'll continue to see him contribute to the United States one way or another."