A federal judge on Thursday vacated the death sentence of Wyoming’s only inmate awaiting capital punishment, in a ruling that faulted the murder convict’s trial lawyers for not adequately presenting to jurors evidence of his mental impairment and troubled life.

Dale Eaton, 69, was found guilty in 2004 in the 1988 kidnap, rape and murder of 18-year-old Lisa Marie Kimmell, whose body was found near Casper, Wyoming, soon after her disappearance. She had been traveling from Colorado to Montana when she stopped in Wyoming and encountered Eaton.

Eaton was behind bars in an unrelated case when investigators in 2001 used DNA analysis to link him to Kimmell’s killing, which was dubbed the “Lil Miss Murder” because of the license plate “LIL MISS” on her car. They found the car buried at Eaton’s former property in Moneta, Wyoming.

During the death penalty phase of his trial, Eaton’s attorneys presented evidence the former welder suffered life-long depression and brain dysfunction and was abused as a child, but jurors still found he deserved the death penalty.

U.S. District Judge Alan B. Johnson found that the defense team’s presentation was inadequate.

“It is the conclusion of this court there is, at a minimum, a reasonable probability [Eaton] would not have received a sentence of death if the mitigating evidence presented this court had been presented to the trial court jury,” he wrote in a 375-page written opinion.

For instance, the public defenders who represented Eaton during his trial failed to properly look into his mental health history and evidence that was available at the time that he suffered from bipolar disorder, Johnson wrote.

The judge’s ruling leaves intact Eaton’s conviction but requires state prosecutors to grant him a new sentencing hearing in court. If they choose not to take that route, he would serve a life sentence without the possibility of parole.

Wyoming Attorney General Peter Michael’s office in a statement said it was disappointed by the decision.

“Wyoming prosecutors recognize the seriousness of capital punishment and seek it in only the most egregious cases. Mr. Eaton’s kidnapping, rape and murder of Lisa Marie Kimmell is one such case,” it said.

Eaton was the only Wyoming inmate on death row, said a representative from Michael’s office. The last inmate put to death in the case was convicted murderer Mark Hopkinson in 1992.