Indonesian authorities say Chicago native Tommy Schaefer admitted that he killed his girlfriend's mother while the family vacationed last month at the upscale St. Regis resort in Bali.

Schaefer's girlfriend, Heather Mack, who is three months pregnant, has also confessed to witnessing her mother's murder and helping to dispose of her body, police in Bali said.

"Both of them have confessed," Bali Regional Police Chief Colonel Djoko Heru Utomo said. "Tommy was the one who carried out the killing."

"Heather thought that Tommy did not mean to kill her mother," Utomo said.

Mack, 19, and Schaefer, 21, have been behind bars since the gruesome discovery of 62-year old Sheila von Wiese-Mack's body, found stuffed inside a silver suitcase left in the trunk of a taxi last month.

Mack previously claimed her mother died during an armed gang attack that she and Schaefer escaped. But Indonesian investigators say surveillance video shows Mack's mother and Shaeffer arguing in the hours before the murder.

Mack's attorney declined a request for comment from ABC News.

According to Bali authorities, Schaefer allegedly killed von Wiese-Mack because he was "hurt and offended" following an argument.

"In Indonesia, you get credit for admitting and cooperating. So he might think this will save him some time if he is ever sentenced," said defense attorney Janet Johnson, who is not involved in the case.

Despite these reported new confessions, the couple has yet to be formally charged.