BOISE, Idaho - The parents of an American prisoner of war captured nearly four years ago in Afghanistan say they have received a letter they believe was written by him.

Army Sgt. Bowe Bergdahl of Hailey, Idaho, disappeared from his base in Afghanistan on June 30, 2009.

His mother and father issued a statement Thursday saying they have received a letter they are confident was written by their son. Bob and Jani Bergdahl said the letter, delivered through the International Committee of the Red Cross, gives them hope that their son is doing as well as can be expected under the circumstances.

"Our family is greatly relieved and encouraged by this letter," they wrote.

They didn't release excerpts from the letter or detail its content.

They thanked the Red Cross and renewed their plea for his captors to release their son, who turned 27 on March 28.

"In the meantime, we ask that you please continue to keep him in good health and allow him to keep corresponding with us," they wrote.

Though he is thought by the United States to be held by the Haqqani Network, a Pakistan-based group that the Obama administration has declared a terrorist organization, leaders of the Haqqani said in December that Bergdahl was being held by another Taliban group.