A 17-year-old girl whose body was found in a suitcase in Thailand died from suffocation, according to authorities.

Police in the coastal city of Pattaya previously said they had found Tunchanok Donhomla’s naked body in a bag near a railway track in the early hours of 27 June.

This was just 15 minutes after they had arrested 45-year-old Australian man Simon Peter Carman, who was allegedly “preparing to flee the country”, they said.

On Wednesday, Pattaya City police chief Anek Srathongyoo told the BBC that Tunchanok’s autopsy results “showed she died from suffocation”.

“She also suffered some brain injury but that was not the cause of death,” he added. “It could have happened after she died as the accused moved her body in a suitcase before dumping it.”

Anek further revealed that the DNA found on Tunchanok’s body matched Carman’s, and suggested that it “likely came from when he put her body in the suitcase”.

Police are pressing four charges against Carman: murder; concealing or moving the body; kidnapping a child over 15 but under 18; and taking away a child over 15 but under 18 for an indecent act.

“[Carman] did not completely deny all the charges initially,” Anek said. “He admitted to taking her out. That said, even if he pleads not guilty in court later, I believe we have collected all the evidence thoroughly and closed every loophole in the case in order to prove it to the court that he has committed those crimes.”

Police will be ready to submit the case to prosecutors by Friday, according to Anek, who noted that the deadline for them to indict Carman was 18 September.

Previously released CCTV footage allegedly showed Carman entering a condominium with Tunchanok at 03:34 local time on 25 June. Later that evening, he emerged alone “carrying a large suitcase”, according to a police statement.

The statement said Carman loaded the bag onto a motorbike before driving towards a railway line.

Following his arrest, Carman denied the charges against him and claimed he had acted in self defence.

He reportedly told police he had agreed to pay Tunchanok 1,000 baht ($30; £23) for sexual services, but they had had an argument when they returned to his apartment and he had offered only 500 baht.

In a video recorded while he was in custody, the suspect issued a message to Tunchanok’s family, saying: “I feel bad for what happened to your daughter. It was out of my control.”

“I know you’ll be very sad, upset… same [as] me,” he added. “Please tell other girls… just to be careful.”

Tunchanok’s family previously told the ABC she was an only child who lived with her father and step-mother in the province of Kalasin, about 480km (298 miles) north-east of Pattaya.

The teen, whose nickname was Cake, had told her parents she wanted to go on holiday with a friend, and had travelled to Pattaya on 16 June.

Her father, Thongchai Donhomla, said he was “deeply saddened” by his daughter’s death.

“My daughter had no mother, so whenever she wanted anything, she would find a way herself, and she always helped me too,” he told a local reporter.

Her stepmother, Oradee Bussarakum, said that the news of Tunchanok’s death had confirmed their fears.

“We were scared. We just hoped it wouldn’t turn out the way we feared,” she said. “Now our eyes are swollen from crying.”

“I just want [Carman] executed,” she added. “I even asked the police if I could hit him, if I could beat him.”

If convicted of murder, Carman could face the death penalty.