JOINT BASE SAN ANTONIO – FORT SAM HOUSTON, Tex. — An attorney for Army Sgt. Bowe Bergdahl said Thursday that a board of psychiatrists concluded that the soldier was suffering from a severe mental defect or disease when he walked off his patrol base in Afghanistan and was captured by the Taliban, providing a potential glimpse into how his legal team may defend him against criminal charges.

The assertion emerged during a preliminary hearing at which an Army officer will consider whether the service should court-martial Bergdahl for abandoning his unit June 30, 2009, in Paktika province. Bergdahl faces up to life in prison after the Army charged him in March with desertion and misbehavior before the enemy.

Bergdahl’s platoon commander in 2009, then 2nd Lt. John Billings, testified that he had no idea that Bergdahl was suffering from mental illness or had previously washed out of the Coast Guard for failing to adapt to life in the service. Billings, now a captain, said that Bergdahl was a good soldier who did not complain, and seemed fine when he encountered him at Observation Post Mest one day before he vanished.

Bergdahl was captured within days by insurgents affiliated with the Taliban, and held until the Obama administration agreed last year to a controversial deal in which he was exchanged for five Taliban detainees held at the military prison at Guantanamo Bay, Cuba. The Afghan detainees were sent to Qatar, and remain there.

The disappearance of Bergdahl triggered an intense period of searching. Maj. Silvino Silvino, the commander of Bergdahl’s company at the time, said Thursday that one soldier in his company survived three explosions within a short period of time while traveling in vehicles searching for Bergdahl. The improvised explosive devices likely caused a concussion, but the soldier was able to return to his unit, Silvino said.

Silvino described his soldiers as growing increasingly frustrated as the weeks past by and there was still no sign of Bergdahl.

“I feared the worst,” said Silvino, “I feared that Sergeant Bergdahl was already captured and they were going to make a run for it and head toward Pakistan.”

Bergdahl was indeed held in Pakistan for much of the following years, until he was released to a U.S. Special Operations team May 31, 2014 as part of the swap, which was approved by the White House. Silvino said his men lived “outdoors in the dirt” for weeks at a time looking for him.

“I would tell them, ‘We need to do this because he is one of us,'” Silvino said. “We don’t know what happened to him. And they were confused by  this.”

The search extended well beyond Silvino’s company of soldiers, but it played a primary role. Several of his mine-resistant vehicles were destroyed by improvised bomb blasts looking for Bergdahl, he said. The number of patrols his unit carried out doubled or tripled in the following weeks.

Silvino said it was “a mark” on his company for Bergdahl to walk away, and that it still stings when other soldiers bring it up.

“I think lesser men would have caved,” he said of the pressure on his soldiers at the time. “But they hung in there. They were extraordinary.”

Col. Clinton Baker, Bergdahl’s former battalion commander, said it is hard to quantify how much violence his soldiers were exposed to as a result of launching Operation Yukon Recovery, the effort to find Bergdahl. But the number of times the soldiers were exposed to explosions and firefights undoubtedly increased, he said.

The soldiers completed a number of dangerous daytime helicopter assaults and operated in the field for weeks at a time, he said.

“There was no battle rhythm. It was just go as hard and as fast as you can all the time,” Baker said. “We completely stopped doing counterinsurgency and focused entirely on the recovery of Sergeant Bergdahl.”

The colonel said he had no doubt his paratroopers could handle the grueling mission physically and mentally. But the operation was difficult on morale.

“Frankly, I felt a bit at a loss with what to do,” Baker said. “My entire time in the Army, I don’t recall that amount of adversity and not being able to overcome it.”