An Army sergeant charged with killing two fellow soldiers pleaded guilty to lesser charges Monday at the start of his court-martial on two counts of premeditated murder.

The murder case against Sgt. Aaron Stanley (search), 23, of Bismarck, N.D., was to proceed after a recess. Stanley is charged with killing Staff Sgt. Matthew Werner, 30, of Oxnard, Calif., and Spc. Christopher D. Hymer, 23, of Nevada, Mo.

The two were shot to death last September at a home in Clay Center, about 30 miles from Fort Riley (search). Military authorities have not revealed a possible motive.

Stanley, who is represented by Army defense counsel, has pleaded not guilty to the murder charges. If convicted, he could face life in prison without parole, said Lt. Col. David Velloney, staff judge advocate.

Also charged is Sgt. Eric Colvin, 23, of Papillion, Neb. A decision on whether he will face a court-martial hasn't been made.

Stanley pleaded guilty to drug use, drug possession, being absent without leave and adultery. The guilty pleas were accepted by Lt. Col. Timothy Grammel, who is presiding in the case.

Stanley admitted having methamphetamines in his possession at his residence and a barracks at Fort Riley, and to distributing the drug on the Army post. He also admitted adultery on two occasions with the wife of another Fort Riley soldier. The AWOL charge stemmed from his being unable to report to duty when he was in jail on drug charges.

Stanley, Colvin and the soldiers who were killed were all part of the 1st Battalion of the 41st Infantry Regiment, 3rd Brigade, 1st Armored Division based at Fort Riley. About 750 soldiers from the unit are serving their second tour of duty in Iraq.