BALI, Indonesia (AP) — An Indonesian court has convicted an American man of murdering his girlfriend's mother and sentenced him to 18 years in jail.

The Denpasar District Court ruled Tuesday that Tommy Schaefer, 21, was guilty of battering Sheila von Wiese-Mack to death in a hotel room on the resort island of Bali. Also on trial is his girlfriend Heather Mack, 19, who is charged with helping in the Aug. 12 murder.

Schaefer and Mack, both from Chicago, are being tried separately in the same court with the same judges and prosecutors.

They were arrested a day after the body of von Wiese-Mack, 62, was found in a suitcase inside the trunk of a taxi at the St. Regis Bali Resort.

"Taking into account Chapter 340 of the criminal code and other related laws, (the panel) declared that the defendant Tommy Schaefer has been legally and convincingly guilty of committing premeditated murder," said the verdict read out by presiding Judge Made Suweda.

"Therefore the defendant is sentenced to 18 years in jail."

He described Schaefer's deeds as sadistic, but said his politeness and an expression of remorse during his trial meant he did not receive a heavier sentence.