If Bowe Bergdahl walked off his base voluntarily before ending up in the hands of the Taliban -- as several of his former fellow soldiers are claiming, it may not have been the first time the then-private first class wandered away in serious violation of military regulations, Fox News has learned.

The exact circumstances that led to Bergdahl's capture in Afghanistan are not known to the public, but a classified 2010 Army investigation concluded he had “willfully walked off base,” two senior defense officials told Fox News. The report, known in the military as a 15-6 investigation, cited testimony from Bergdahl’s fellow soldiers who said he had walked off the base alone before -- out of “boredom” – according to this official.

Still, the report stopped short of calling Bergdahl a deserter, at least in part because Bergdahl had not yet been interviewed in order to determine his intent.

“You can’t have desertion without intent,” one official said.

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Bergdahl, 28, who was turned over to the U.S. on Saturday in exchange for five top-level Taliban operatives, was held for five years after being captured June 30, 2009, near the eastern Afghan province of Paktika. He is believed to have been held by the ruthless Haqqani network, in both Afghanistan and Pakistan during his captivity.

The deal for his release has rekindled questions about whether he was abducted or left voluntarily. In emails to his father first reported in 2012 by Rolling Stone, Bergdahl expressed his disillusionment with the Army and America in general. Fellow soldiers, incensed that Taliban terrorists held at Guantanamo Bay were exchanged for Bergdahl, have come forward in recent days to say he was a deserter and that the subsequent search for him resulted in troop deaths.

Former Army Sgt. Evan Buetow, who was Bergdahl's team leader, told Fox News Channel Wednesday that Bergdahl acted strangely before his disappearance. He asked for help mailing his computer and belongings home, and asked what would happen to him if his military gear turned up missing.

"Him walking away from the base, that wasn't surprising to me," Buetow said. "Things that he said to me personally, things that he said to others in the platoon, they were not that shocking or alarming at the time. But when he walked away, that light bulb went off, and it was like, "Yep, he walked away.

"In the days following, with all the assets we had in the area looking for him, we heard over the radio chatter that there was an American looking for someone who spoke English so he could talk to the Taliban. I heard that right over the radio and right from the interpreter's lips when he said it. That alarmed everybody. We knew at that point that he had walked away, but he was attempting to contact the Taliban of his own accord."

Earlier this week, two American soldiers who served in a platoon alongside Bergdahl told Fox News’ Megyn Kelly they also believe Bergdahl deliberately walked off base, with one saying he does not want to see his former comrade “hailed as a hero.”

Gerald Sutton and Cody Full were serving in Afghanistan with Bergdahl when he was captured by the Taliban. Full told Kelly on “The Kelly File” that there is no doubt in his mind that Bergdahl deserted.

Full said there were “telltale signs” that Bergdahl deserted before he was captured, including that he mailed his possessions home, expressed disillusionment with the war in emails to his family and talking extensively about the surrounding terrain.

“(Bergdahl was) talking extensively to the Afghan national police in a way that was not about hearts and minds, there was an agenda there when he spoke to them,” he said.

Sutton told Kelly that he also believes Bergdahl’s disappearance was “premeditated.”

“His true motives and everything, I really don’t know he never spoke about them,” Sutton said.

After weeks of intensive searching, the military decided against making an extraordinary effort to rescue Bergdahl, especially after it became clear he was being held in Pakistan under the supervision of the Haqqani network, a Taliban ally with links to Pakistani's intelligence service. However, the U.S. government kept tabs on Bergdahl's whereabouts using spies, drones and satellites, even as it pursued off-and-on negotiations to get him back.

Also calling into question Bergdahl's motives were emails between him and his father, published in by Rolling Stone in 2012, in which he told his father of his growing disenchantment with the Army's mission in Afghanistan.

“The future is too good to waste on lies,” Bergdahl wrote. “And life is way too short to care for the damnation of others, as well as to spend helping fools with their ideas that are wrong. I have seen their ideas and I am ashamed to even be American. The horror of the self-righteous arrogance that they thrive in. It is all revolting.”

"OBEY YOUR CONSCIENCE," was the subject line of an e-mail reply from Bob Bergdahl to his son, just three days before he disappeared from the base.

The Washington Post also reported Wednesday that Afghan villagers who saw Bergdahl after he left his base said he seemed to be heading toward Taliban strongholds.

Ibrahim Manikhel, the district’s intelligence chief, also told the Post that the villagers warned him not to go to those “dangerous” areas, but he went anyway.

“We think he probably was high after smoking hashish,” Manikhel said, according to the Post. “Why would an American want to find the Taliban?”

During Bergdahl's captivity at least four hostage videos were released of him, the most recent in December. He is now at Landstuhl Regional Medical Center, in Germany, receiving medical treatment and is expected to be taken to Brooke Army Medical Center in San Antonio, Texas, at a date not determined.

Although the 2010 report is classified, Secretary of the Army John McHugh said the Army has started a new review of the circumstances surrounding Bergdahl's disappearance. That probe will seek to answer questions about the intentions of Bergdahl, who hails from Idaho and who was promoted to sergeant while in captivity.

Army Chief of Staff Raymond Odierno said Wednesday that now that Bergdahl is safe, there will be time and a process to determine what exactly happened during his disappearance.

"Now that Sgt. Bergdahl is back and under our control, first and foremost we must ensure his health is taken care of and he is properly reintegrated," Odierno said. "At the appropriate time, we will conduct a thorough, transparent and complete review of the circumstances surrounding his capture."

Fox News' Justin Fishel and Jennifer Griffin contributed to this report