(ABC News)

A 2009 preliminary investigation of Bowe Bergdahl's disappearance in Afghanistan noted he had briefly left his unit on two prior occasions before rejoining his fellow soldiers, once in the United States and a previous time in Afghanistan.

U.S. officials also indicate that Bergdahl may have escaped from his captors more than once before being recaptured, though the U.S. has never been able to definitively confirm that the escapes took place.

While training for his unit's deployment to Afghanistan, Bergdahl had briefly walked away from his unit while it was conducting training at the National Training Center at Fort Irwin, California, located in the Mojave Desert, according to a U.S. official briefed on the contents of that investigation said.

The investigation also found that Bergdahl had wandered away from his unit while deployed to eastern Afghanistan prior to his eventual disappearance on June 30, 2009, the official said.

Concluded in August 2009, the investigation determined that Bergdahl had likely walked away on his own from his unit's remote outpost. It was also critical of discipline and security practices within in his platoon.

There is no determination in the report as to whether Bergdahl intended not to return to the outpost on the night he was captured by the Taliban.

Bergdahl may have tried to escape from his captors on more than one occasion, though the details remain murky and the U.S. was never definitively able to confirm potential escape attempts, given it did not have precise information about his whereabouts. A former official with knowledge of the intelligence at the time said unconfirmed information about the escape attempts were based on intercepted Taliban "signal intelligence," likely a radio or cellphone transmission.

The Obama administration has been criticized for securing Bergdahl's freedom in exchange for the transfer of five high-profile Taliban figures who had been at the detention camp in Guantanamo Bay. There has also been harsh criticism that the administration did not follow legal requirements to notify Congress of a pending release of detainees from Guantanamo.

The administration has begun briefing members of Congress to make their case that Bergdahl's condition was deteriorating and warranted quick action. The briefings include a "proof of life" video secured from the Taliban earlier this year.

A U.S. official said the U.S. had received two short proof of life videos provided by the Taliban through intermediaries in Qatar. Neither video has ever been made public and remain classified.

Unlike five previous videos released by the Taliban in the two years after Bergdahl was captured, the brief video clips were intended to demonstrate Bergdahl's condition to U.S. officials.

The official said the first video was delivered in December 2011 and was provided to the U.S. by Qatari intermediaries. A second and shorter proof of life video was received by the United States two years later in January 2014. It is believed to have been recorded in December 2013 as Bergdahl made a mention of Nelson Mandela's recent passing.

That video was also provided via Qatari intermediaries. American officials have said that the most recent video was the one that raised the most concern about his condition.

A senior administration official who has seen the second proof of life video said Bergdahl, with a beard, was quivering slightly, and one arm was holding the other. He was speaking haltingly in a raspy voice, and was clearly under duress, the official said.

But some senators who viewed the video at a classified congressional briefing Wednesday night said they were not convinced that Bergdahl's condition warranted the release of five senior Taliban detainees.

"It appeared that he was drugged, and that he was barely responsive in the video itself," said Sen. Saxby Chambliss, the ranking Republican on the Senate Intelligence Committee. Critical of the Bergdahl swap, Chambliss told reporters, "I don't think from a health standpoint there was any issue that dictated the release of these five nasty killers in exchange for Bergdahl."

Sen. Joe Manchin, D-West Virginia, also said he was not convinced that Bergdahl's life was in danger based on what he saw in the video.

"That did not sell me at all. The proof of life was basically five months ago. December? At that time he was impaired," Manchin said. "That is not the person that was released here. He was not in that type of dire situation when he was released."

Bergdahl continues to receive reintegration treatment at the Landstuhl Regional Medical Center in Germany where he is expected to remain until next week when he is transferred to another Army medical facility in San Antonio, Texas.

Officials say he is still having difficulty expressing himself in English, particularly with his vocabulary and recovering words, though his comprehension is said to be high.

ABC News' John Parkinson contributed to this report.