An American couple arrested on the Indonesian resort island of Bali in connection with the death of the woman’s mother could face charges of premeditated murder carrying a maximum penalty of death, police said on Saturday.

Heather Mack, 19, and her boyfriend, Tommy Schaefer, 21, both from Chicago, were arrested in Bali’s Kuta area Wednesday, a day after the body of Sheila von Wiese-Mack was found stuffed inside a suitcase in the trunk of a taxi parked in front of the upscale St Regis Bali Resort in the island’s exclusive Nusa Dua section.

An autopsy on Saturday found that Von Wiese-Mack, 62, died of asphyxiation from a broken nose bone resulting from a blunt blow, said Ida Bagus Putu Alit, head of forensics at Sanglah Hospital in Bali’s provincial capital of Denpasar. She also suffered a broken neck, Alit said.

Mack and Schaefer were charged with murder on Friday. Colonel Djoko Hari Utomo, chief of police for Denpasar, said on Saturday that police had enough evidence to charge them with premeditated murder.

He reiterated that the couple was still refusing to talk to investigators without the presence of American lawyers.

“But that does not hamper our investigation. We have enough evidence, including CCTV and fingerprints,” Utomo said. “And I am inclined to charge them with premeditated murder.”

An FBI agent arrived on Friday to help with the investigation, including on the couple’s possible involvement in criminal activity in the US, Utomo said.

The autopsy found that breaks in Von Wiese-Mack’s neck and nose extended to her upper right and left jaws, causing respiratory disorders, Alit said. It also showed hand wounds suggesting she was trying to fend off an attack.

“We also found blood aspiration, which meant the victim was standing when assaulted,” Alit said. “The conclusion is that the victim suffocated from lack of oxygen because of influx of blood from the broken nose bone.”

Alit said Von Wiese-Mack was believed to have died on Tuesday morning.

“The victim was last spotted on CCTV at 3.45am, and three hours later she called receptionists asking to wake her up at 10am,” Alit said. “However, she did not respond when awakened.”

Meanwhile, Bali’s deputy police chief, Brigadier General Gusti Ngurah Raharja Subyakta, said psychological tests showed there were no psychiatric disorders with the couple.

Von Wiese-Mack, also from Chicago, and her daughter arrived at the St Regis on 9 August, while Schaefer checked in on Monday, two days later, according to police. Security camera video showed the victim had an argument with Schaefer on Monday in the hotel’s lobby.

Police say Mack and Schaefer placed the suitcase inside the trunk of the taxi and asked the driver to wait while they checked out of the hotel. However, they didn’t return, and hotel security guards who had found blood spots on the suitcase suggested the driver take the taxi to a police station, where officers opened the suitcase and discovered the body.

The couple told investigators that Von Wiese-Mack was killed by robbers while they managed to escape, Utomo said.

Authorities in an upscale Chicago suburb, Oak Park, have examined records of 86 incidents in which police were called to the house where Von Wiese-Mack lived with her daughter. Friends have alleged that the mother-daughter relationship was sometimes contentious.

Von Wiese-Mack was the widow of highly regarded jazz and classical composer James L Mack, who died in 2006 at age 76.