The family of a U.S. prisoner of war captured nearly four years ago in Afghanistan said it received a letter it believes was written by him.

U.S. Army Sgt. Bowe Bergdahl disappeared from his base on June 30, 2009, and is believed held in Pakistan.

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 in a statement issued Thursday.

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

They renewed their plea for his captors to release Bergdahl, who turned 27 on March 28.

“We hope Bowe’s captors will again consider his parents’ plea to release him, but 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.

Col. Tim Marsano, a National Guard spokesman acting as a media liaison for the Bergdahl family, said details from the letter indicate it’s genuine.

Though he hasn’t seen the letter, it was his understanding that it was a physical letter and had been handwritten.

Bergdahl’s captivity has been marked by only sporadic releases of videos and information about his whereabouts.

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

The International Committee of the Red Cross seems to be acting as a go-between for Bergdahl and his family. Part of its mission is to protect victims of armed conflict and other violent situations and provide them with assistance.

“We talk to all the parties in a conflict … to make sure the detainees are kept in good conditions, fed properly, taken care (of) if they have any medical need,” said Alexis Heeb, a spokesman in Geneva. “Of course, one of the priorities, whatever a detainees’ status, is they should have contact with their families. For us, it’s essential that detainees stay in touch with their relatives, wherever they may be in the world.”