JEFFERSON CITY, Mo. — A man sentenced to death for murdering a 6-year-old he abducted from her father’s St. Louis County home sought a new trial Wednesday, claiming his attorneys should have pursued a defense that he suffered from brain damage.

Johnny Johnson has admitted that he killed Cassandra “Casey” Williamson in July 2002, though attorneys at his trial said mental illness made him incapable of acting with “cool reflection” and he thus shouldn’t have been eligible for the death penalty.

During appeal arguments Wednesday to the state Supreme Court, a new attorney for Johnson argued that his trial attorneys were negligent for not hiring a neuropsychologist who could have testified that Johnson suffered from brain damage in addition to his mental illnesses. Johnson is seeking a new trial, or at least a new sentencing hearing.

“The jury heard only half the story — the mental disease. There was nothing about the mental defect,” said Bob Lundt, an attorney in the St. Louis public defender’s office who is representing Johnson.

He told the Supreme Court that Johnson suffered three head injuries as a child and two more as an adolescent. Lundt said those made it difficult for Johnson to deliberate about his actions.

But under questioning from the judges, Lundt said no brain scan could show the injury and no scientific evidence could specifically say such brain injuries cause people to commit murder.

Assistant Attorney General Shaun Mackelprang argued that Johnson’s trial attorneys made a logical and strategic decision in focusing on the mental illness as a defense. He said neurological tests conducted on Johnson after his conviction were subjective and Johnson could have intentionally performed poorly in hopes of winning a new trial.

Among those watching the Supreme Court arguments were Casey’s mother, aunt, grandmother and several other relatives or family friends.

Della Steele, who said she was Casey’s great-aunt, said she also had watched Johnson’s original trial and believes he is mentally ill. But she said she still believes he made a choice to kill Casey and should bear the consequences.

“Him being executed is not going to bring Casey back, but what it can do is protect the children of our society — to make sure he never has access to a child again,” Steele said.

Johnson, who was 24 at the time of the crime, admitted he took Casey on a piggyback ride from the home where he had been staying as a transient guest for a few days and then crushed her heard with bricks and rocks after she resisted his attempts to rape her. The killing happened at the ruins of an old glass factory in the St. Louis suburb of Valley Park.

Johnson was convicted of first-degree murder, armed criminal action, kidnapping and attempted rape. In addition to the death sentence, he received three consecutive life prison terms.

Since Casey’s death, her family has undertaken various initiatives in her memory, including a safety fair for parents and children and fundraisers for college scholarships. Steele said the family’s goal is to raise enough money to give a scholarship to each of the graduating members of what would have been Casey’s senior class from Valley Park in 2014.