A mentally ill U.S. soldier accused of killing an Afghan detainee will plead guilty to premeditated murder in exchange for a substantially reduced prison term, his lawyer said Sunday.

Pfc. David Lawrence of Lawrenceburg, Ind., is expected to enter the plea at a military court hearing at Fort Carson, Colo., on Wednesday, said James Culp, Lawrence's civilian attorney.

Lawrence, 20, could have faced up to life in prison or execution. Culp declined to say how long a prison term Lawrence would be ordered to serve under the plea agreement. A Fort Carson spokeswoman didn't immediately return a call seeking comment Sunday.

The plea deal was first reported by the Los Angeles Times.

Lawrence is accused of fatally shooting a suspected Taliban member who was asleep in a jail cell at a U.S. outpost in Afghanistan on Oct. 17.

Lawrence, assigned to Fort Carson's 1st Brigade Combat Team, 4th Infantry Division, was taking antidepressants at the time after seeking help from an Army combat stress center in Afghanistan. He was assigned to guard duty when he returned from the stress center to his unit in in the Arghandab Valley just outside the city of Kandahar.

An Army Sanity Board concluded in January that Lawrence had schizophrenia and post-traumatic stress disorder at the time of the shooting. The board said Lawrence was "unable to appreciate the nature and quality or wrongfulness of his conduct at the time of the alleged criminal misconduct."

The board said Lawrence was capable of understanding the procedure against him and of helping his lawyer.

Army officials echoed that finding when they announced in January they still intended to court-martial Lawrence. "The government feels it can proceed with trial at general court martial and the accused can cooperate intelligently in his defense," officials said in a written statement at the time.

Culp and others with experience in the military judicial system said at the time that the Army rarely prosecuted soldiers after such a finding. But the case comes as the Army struggles to address mental health problems in the ranks while fighting two protracted and draining wars.

U.S. commanders also have been contending with complaints from Afghan officials about the conduct of troops in the U.S.-led coalition.

Lawrence's father, Brett Lawrence, and Culp described the soldier as a troubled young man from a family with a history of schizophrenia.

Before the shooting, he was badly shaken by the deaths of two friends in Afghanistan, including a chaplain, according to his lawyer and his father. Brett Lawrence also said his son had told family members before the shooting that he was hearing voices.

The soldier asked for and received help from mental health providers in Afghanistan before the shooting, according to testimony at a hearing at Fort Carson last fall.

But when he returned to his unit in the Arghandab Valley, he wasn't given the care or supervision he needed, Culp said after that hearing. Instead, Culp said, he was assigned to longer-than-normal shifts on guard duty.

He was on guard duty at the cell of the prisoner he is accused of shooting, identified in Army documents only as Mohebullah, who was awaiting transfer to NATO coalition custody.

Prosecutors said Lawrence didn't tell Army officials until after the shooting that he was hearing voices. Brett Lawrence said that would be understandable because his son had seen an aunt, an uncle and a grandmother all subjected to ridicule when they spoke of hearing voices.

Brett Lawrence said Sunday he is happy his son won't spend the rest of his life in prison.

He declined to disclose the length of the proposed prison term, but he said he believes it would be the shortest the military has ever imposed in a premeditated murder case.

He said his son is under guard in a Fort Carson barracks and is on medication prescribed by military doctors.

"He is doing better, knowing what's going to happen to him," the father said.