YOKOHAMA, Japan — The prosecution in the Olatunbosun Ugbogu robbery-homicide case was dealt a small blow Thursday afternoon when the three-judge panel ruled that evidence from a psychiatrist is admissible in the trial.

The court will appoint a doctor to document the 22-year-old U.S. Navy seaman’s behavior and report his findings. He was expected to testify Friday.

The motion’s approval is a boost to Ugbogu’s insanity defense. A guilty verdict on all charges could bring the former USS Cowpens sailor the death penalty or life imprisonment in Japan.

Yasutoshi Murakami, his attorney, told the court Ugbogu is mentally ill and that’s what led him to fatally stab 61-year-old taxi driver Masaaki Takahashi in an alley in Yokosuka’s Shioiri neighborhood last spring. A psychiatric evaluation is necessary for "partiality and fairness," he added.

"I really appreciate what the judge ordered because … U.S. military personnel also have the right to get a fair trial," Murakami said after Thursday’s session. "The suspect suffers from mental illness. He stated he was ordered by voices to stab someone."

On Thursday, prosecutors argued against allowing a mental health expert to hear evidence or enter the proceedings on Ugbogu’s behalf, saying he should be held criminally responsible for the murder.

But the lead judge said a psychiatrist should be permitted to provide testimony "in light of the gravity of this case." Yokohama Detention Center officials report that Ugbogu suffers from schizophrenia and has been taking medication, according to the judge.

"We will take an expert medical opinion based on interviews and sessions with the defendant," the judge said.

Also Thursday, Murakami read from some of Ugbogu’s writings and showed drawings he purportedly sketched in his detention cell.

In one handwritten note, Ugbogu pleaded for his lawyer’s help, stating he was still "tormented by voices." A drawing in comic strip form detailed more encounters with the so-called "spirit friends" and included the sailor’s account of the events leading up to the March 19 killing.