Former U.S. base worker Kenneth Franklin Shinzato on Tuesday appealed a court ruling that sentenced him to life in prison for killing a 20-year-old Okinawa woman while attempting to rape her.

The Naha District Court handed down the 33-year-old Shinzato’s sentence on Dec. 1, saying the April 2016 attack was carried out with “selfish motivation, which left no room for leniency.”

Shinzato admitted during the lay judge trial that he assaulted the woman with the intention of raping her and to abandoning her body but denied he intended to murder her.

According to the ruling, Shinzato attacked the woman on a road in the central Okinawa city of Uruma on the night of April 28, 2016. He struck her on the head with a bar and stabbed her in the neck with a knife so she would not resist. The attack was so violent that she died from her injuries.

Defense lawyers argued that the woman’s death may have been caused by her hitting her head on the ground, but the district court ruled that he intended to commit murder because the attack was of such severity that it was likely to kill.

The case sparked public uproar and strengthened anti-U.S. base sentiment in Okinawa, which hosts the bulk of U.S. military facilities in Japan and has seen a series of crimes committed by U.S. servicemen or military-linked personnel.

Shinzato was a civilian working for an internet company on the U.S. Kadena Air Base at the time of the attack. He served as a U.S. Marine from 2007 to 2014, according to his lawyers and the U.S. Defense Department.