Fort Riley ? A fellow soldier involved in illegal drug trafficking with an Army sergeant testified Tuesday during a court-martial that the sergeant killed two other soldiers because he thought they were informants for post police.

Sgt. Eric Colvin acknowledged that he hoped to clear his own name by testifying against Sgt. Aaron Stanley. Defense attorneys attacked Colvin’s truthfulness and questioned whether psychological problems and drug use impaired his memory.

Stanley faces two counts of premeditated murder for shootings last September at his home in Clay Center, about 30 miles west of Fort Riley.

“It’s a tragedy that two men lost their lives,” Colvin said. “This is the way that justice will be brought to the families.”

Colvin, 23, of Papillion, Neb., testified under a plea agreement with prosecutors, and the post hasn’t decided whether he will face a court-martial himself. Officials haven’t made details of the agreement public.

Stanley, 23, of Bismarck, N.D., has acknowledged shooting the two soldiers, but his attorneys contend he did so to defend himself and Colvin. The victims were Staff Sgt. Matthew Werner, 30, of Oxnard, Calif., and Spc. Christopher D. Hymer, 23, of Nevada, Mo.

In previous proceedings, defense attorneys have attempted to portray Colvin as the leader of the drug trafficking operation.

During cross-examination, Stanley’s lead attorney, Capt. Anita Robbins, asked Colvin, “After the deal got better, your memory got better?”

Colvin also acknowledged that he suffered from post-traumatic stress disorder, which is brought on by traumatic experiences such as combat and can cause psychological problems, including flashbacks and memory loss. He said he was hospitalized during April 2004, after returning from a tour in Iraq, and acknowledged suffering blackouts.

“There are certain parts that are hazy,” he said. “But I have a good assessment of what did happen.”

He also admitted using methamphetamines, which he said resulted in him having “a lot of crazy ideas in my mind.”

Military prosecutors contend Stanley killed Werner and Hymer to prevent post police from discovering their methamphetamine manufacturing and marijuana growing. Prosecutors have told the eight-member panel hearing the case that said Stanley’s growing drug use made him paranoid.

Colvin said Stanley suspected even him of being an informant, confronting him two days before the shooting.

Stanley pleaded guilty Monday to drug use, drug possession, being absent without leave and adultery. His sentencing on those charges has not been set.

The AWOL charge stemmed from Stanley being unable to report to duty when he was in jail on drug charges.

Stanley’s attorneys said he shot Werner and Hymer because Werner was threatening Colvin with a big kitchen knife, angry over rumors that his wife was having an affair with Stanley. The adultery charge against Stanley did not involve her.

In an interview with the Kansas Bureau of Investigation, Colvin initially corroborated that version of events. During his testimony Tuesday, Colvin said he lied because, “I didn’t want to see my buddy go down for two murders.”

All four soldiers were 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.

Stanley and Colvin both were with Bravo Company of 1st Battalion, 41st Infantry, and had served in Iraq when the unit was deployed for the early stages of the war. Stanley was scheduled to return for a second tour in June 2004, but was being held in Pottawatomie County on eight drug charges.