A US Army investigator began formal questioning of Sergeant Bowe Bergdahl at Fort Sam Houston in San Antonio, Texas, said an Army spokeswoman, Lieutenant Colonel Alayne Conway.
The probe, which has no set release date, is looking into how exactly Bergdahl went missing from his combat post in Afghanistan in 2009.
The determinations range from finding that Bergdahl did nothing wrong to court martialing him as a deserter a charge that in theory carries the death penalty, although execution would be highly unlikely.
The Army appointed Major General Kenneth Dahl, who has deployed to Afghanistan, to interview the 28-year-old after President Barack Obama came under fire for releasing five Taliban prisoners from Guantanamo Bay to Qatar in exchange for Bergdahl's release in May.
Obama has defended the deal, saying it was a long-time, iron-clad principle for the United States to secure the release of its prisoners of war.
Some soldiers have alleged that Bergdahl walked out of his unit willingly, putting other troops at risk as they sought his release, leading to outrage in some circles against the deal struck for his release.
Bergdahl is being defended by prominent lawyer Eugene Fidell, who teaches at Yale Law School.
Fidell, while declining to discuss the case in depth publicly, has said that Bergdahl was proud to wear the US uniform and has been made a scapegoat by people opposed to closing down the controversial prison camp in Guantanamo Bay, Cuba.
After undergoing health exams, Bergdahl has returned to duty by working an office job at Fort Sam Houston.