Randal McCloy (r) and his wife, Anna (2001 photo)

The lone survivor of this week's coal mine accident in the U.S. state of West Virginia is undergoing medical treatment, but prospects for his recovery are not yet certain.

Doctors treating Randal McCloy say he is in a medically-induced coma while he receives pure oxygen, in an attempt to flush poisonous carbon monoxide out of his system. Experts say it is not clear whether this will help because it has been several days since his exposure to the gas.

Meanwhile, the family of one victim has made public a goodbye letter he wrote that was found in the mine. The miner, 51-year-old Martin Toler, Jr. wrote that he was just going to sleep, and promised to see them later in heaven.

U.S. officials are investigating the coal mine accident, in which 12 workers died after an explosion in the mine shaft trapped them underground.