ABC News' Alex Perez reports:

Police in Ohio hope an anonymous apology letter might help solve the month-long mystery of who killed a former Marine and father-of-two who was shot to death while hunting.

Larry Bradley, 45 and a veteran of the Iraq War, was shot in the back and left for dead while hunting deer in Gallia County, Ohio, last month.

The case went cold until around Christmas when the Gallia County Sheriff's Department received a handwritten letter of apology from someone confessing to be the person who killed Bradley.

"I am so sorry for the hurt and pain that I caused on that day…can u please tell the family I am so sorry," the letter reads in part.

Authorities say they will not release the full apology letter in order to protect evidence. They do say there is enough detail in the letter to make them believe it is real.

"The first thought that I thought was that person was really struggling," Gallia County Sheriff Joe Browning said. "Because it is hunting, there is a high likelihood that it was an accident."

The letter reportedly brings comfort but not closure to Bradley's wife, Denise, whom he called as he lay dying.

"He asked her to contact some other family members to bring help for him," Browning said. "I think he didn't want to panic her."

Authorities hope the person who shot Bradley will now turn themselves in. "We owe it to the family to try to locate this person and have them come forward," he said.