YOKOHAMA, Japan — U.S. Navy Seaman Olatunbosun Ugbogu should be imprisoned for life for fatally stabbing a taxi driver last year, a prosecutor said Wednesday during closing arguments in Yokohama District Court.

Prosecutor Tomohito Imamura said Ugbogu planned to rob and kill taxi driver Masaaki Takahashi, 61, after a ride from Tokyo to Yokosuka on March 19, 2008.

"The crime was calculated, cold-hearted and extremely cruel," Imamura said.

Imamura also read a statement from Takahashi’s brother asking the court to sentence Ugbogu to death.

"My anger and hatred toward the criminal, who took [my brother’s] life instantly without hesitation has not changed," the statement read.

Imamura discounted defense arguments that Ugbogu, a Nigerian national, suffers from a mental illness that led him to stab the driver in the neck.

During a hearing in April, a court-appointed professor of neuropsychiatry testified that while Ugbogu may be suffering from dissociative amnesia, he did not believe that Ugbogu heard the voices in his head that he claimed led him to kill Takahashi.

However, defense attorney Yasutoshi Murakami questioned the reliability of the professor’s assessment during his closing argument Wednesday.

The professor lacked necessary background information on Ugbogu’s previous life trauma to make an informed diagnosis, Murakami said.

The prosecutor said Ugbogu planned to rob the driver and use the money for entertainment. But Murakami countered that Ugbogu never took any money, though he did not pay the 19,560 yen (about $200) taxi fare.

Ugbogu previously said he believed at the time of the stabbing that the voices in his head could have hurt him or his family if he did not stab the taxi driver.

Prosecutors cited earlier testimony Wednesday that Ugbogu’s methodical actions and directions to the taxi driver prior to the stabbing did not fit the profile of a schizophrenic.

At an earlier hearing, the court denied Murakami’s motion to allow a defense psychiatric witness.

Ugbogu has been taking antipsychotic medication regularly while in confinement in Japanese jail.

"What the defendant needs is treatment, not punishment," Murakami said.

Ugbogu declined to speak to the court following the concluding arguments, when defendants usually apologize to the court.

Although he apologized briefly at his last hearing, the Japanese system traditionally places importance on multiple apologies.

The court is scheduled to render a verdict July 30.