DENVER - A US Army private deemed mentally ill by psychiatrists was sentenced to 12 1/2 years in prison on Wednesday after pleading guilty to murdering a Taliban commander who was shackled inside a jail cell in Afghanistan last October.

Private First Class David Lawrence, 20, was demoted, dishonorably discharged and sent to prison after the hearing at Fort Carson, Colorado, a base spokeswoman said.

Lawrence was court-martialed for killing Mullah Mohebullah with his M-4 rifle as the detainee was confined to a cell in Arghandab, Afghanistan, on October 17, 2010.

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Lawrence said in open court he shot dead Mohebullah to avenge the killing six weeks earlier of an Army chaplain and other soldiers in his unit in Kandahar province.

US Army Colonel Mark Bridges, the presiding judge, initially sentenced Lawrence to life in prison, then reduced his term to 12 1/2 years. He will be eligible for parole in four years.

Army-appointed psychiatrists who examined Lawrence concluded that he suffered from schizophrenia and post-traumatic stress disorder.

"(Lawrence) was unable to appreciate the nature and quality or wrongfulness of his conduct" when he unlocked Mohebullah's cell and shot him in the face, according to court documents.

Despite the diagnosis, prosecutors decided to move ahead with the court-martial.

On a website documenting the case, Lawrence's relatives said the family has a history of schizophrenia, and that Lawrence sought help from the Army after hearing voices. He was sent back to his unit after he was prescribed two psychiatric drugs.

"With pills in hand and nothing else, he was sent back to a new task of guard detail after less than a week in a combat stress facility," the family said.