JOINT BASE LEWIS-McCHORD, Wash. — Defense lawyers Monday tried to introduce evidence that an Army sergeant charged with the premeditated killing of five fellow service members in Iraq may have had brain abnormalities.
A military judge heard testimony from a specialist who said Sergeant John Russell had ‘‘significant abnormalities’’ in his brain structure and function. Dr. Ruben Gur said the abnormalities indicate Russell could have had difficulty regulating emotions.
But specialists called by the government suggested Russell’s brain is normal. Captain Durward Johnson, an Army prosecutor, said the brain examinations cited by the defense were not pertinent to the case because they were conducted nearly four years after the killings, and the results were compared to the brains of younger people who were not on medication.
‘‘It doesn’t fit,’’ Johnson said.
The judge, Colonel David Conn, did not immediately rule whether the testing would be allowed in the court-martial.
Russell has already pleaded guilty to the 2009 killings but does not agree they were premeditated. Prosecutors are trying to prove that Russell plotted the shootings.
A previous plea agreement means Russell will avoid the death sentence. His maximum sentence would be a life term.
The shooting was one of the worst instances of soldier-on-soldier violence in the Iraq war and raised questions about the mental health problems for soldiers caused by repeated tours of duty.
Russell, who is from Sherman, Texas, is being held at Joint Base Lewis-McChord, about 40 miles south of Seattle.
A hearing on possible charges was held in August 2009 at Fort Leavenworth, Kan. Two evaluations presented during that hearing said Russell suffered from severe depression with psychotic features and chronic post-traumatic stress disorder. A March 2011 evaluation said the major depression with psychotic features was in partial remission.
Russell was nearing the end of his third tour when his behavior changed, members of his unit testified in 2009. They said he became more distant in the days before the May 11, 2009, attack, and that he seemed paranoid that his unit was trying to end his career.
On May 8, Russell sought help at a combat stress clinic at Camp Stryker, where his unit was located. On May 10, Russell was referred to the Camp Liberty clinic, where he received counseling and prescription medication.
Witnesses said they saw Russell the following day crying and talking about hurting himself. He went back to the Camp Liberty clinic, where a doctor told him he needed to get help or he would hurt himself. Russell tried to surrender to military police to lock him up so he wouldn’t hurt himself or others, witnesses said.
Military prosecutors say Russell left the clinic and later returned with a rifle he took from his unit headquarters and began firing.
The shooting killed Navy Commander Charles Springle of North Carolina; Private First Class Michael Edward Yates Jr. of Maryland; Dr. Matthew Houseal of Texas; Sergeant Christian Bueno-Galdos of New Jersey; and Specialist Jacob Barton of Missouri.