WASHINGTON — A Pentagon investigation concluded in 2010 that Army Sgt. Bowe Bergdahl walked away from his unit, and after an initial flurry of searching the military decided not to exert extraordinary efforts to rescue him, according to a former senior defense official who was involved in the matter.

Instead, the U.S. government pursued negotiations to get him back over the following five years of his captivity — a track that led to his release over the weekend.

Bergdahl was being checked and treated Monday at a U.S. military hospital in Germany as questions mounted at home over the swap that resulted in his freedom in exchange for the release of five detainees who were sent to Qatar from the U.S. prison at Guantánamo, Cuba.

The five terrorist suspects also walked free, stirring a debate over whether the exchange would heighten the risk of other Americans being snatched as bargaining chips and whether the released detainees — several senior Taliban figures among them — would find their way back to the fight.

U.S. officials said Sunday that Bergdahl’s health and safety appeared in jeopardy, prompting rapid action.

Questions persisted about the circumstances of Bergdahl’s 2009 capture. Defense Secretary Chuck Hagel declined to comment on earlier reports that the sergeant had walked away from his unit, disillusioned with the war.

Such matters “will be dealt with later,” Hagel said.

But the former Pentagon official said it was “incontrovertible” that he walked away from his unit.