(NEW YORK) -- It's been four years since the disappearance and capture of Army Sgt. Bowe Bergdahl.
Bergdahl, 27, of Hailey, Idaho, was discovered missing during a unit roll-call in Afghanistan on June 30, 2009. He was declared captured by the Taliban four days later.
He is the only known American soldier held captive since the U.S. invasion of Afghanistan in 2001.
Despite the fact that little news has come of Bergdahl's condition, he is believed to be alive.
His parents reportedly received a handwritten letter from their son through the Red Cross in April. While Bergdahl has appeared in Taliban videos since he was taken captive, the last recording showing him alive was in November 2011.
It is believed that Bergdahl is being held in the mountains somewhere along the Afghanistan-Pakistan border by the Haqqani network, a heavily armed, organized and notoriously dangerous wing of the Taliban.
A private first class at the time of his capture, Bergdahl was assigned to the 1st Battalion, 501st Parachute Infantry Regiment, 4th Brigade Combat Team, 25th Infantry Division, based at Fort Richardson, Alaska, before deploying to Afghanistan. He was promoted to specialist while in captivity.
The International Security Assistance Force sent its thoughts and prayers to Bergdahl's family and friends.
"Four years later, we are still waiting for Sgt. Bergdahl's safe return, and it is my sincere hope that the wait will soon come to an end," said ISAF Commander Gen. Joseph F. Dunford in a statement. "To Sgt. Bergdahl's family, I want to say that we know you have not given up hope, and neither have we."
ABC News' attempts to reach Bergdahl's parents on Sunday were not immediately successful.
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