Jomtien, Thailand: Australian Simon Peter Carman went about the ordinary business of shopping, eating out and doing his laundry as a teenage girl he is accused of murdering lay stuffed inside a suitcase for hours in his room in a coastal Thai town, police allege.

Fast detective work and a network of CCTV cameras that captured the 45-year-old’s seemingly casual comings and goings on Thursday led to Carman’s arrest at Bangkok’s Suvarnabhumi Airport the following day, just minutes before he was to board a flight to Australia.

“When he was being detained by immigration officers at the airport, they noticed the [apparent scratch] injuries,” Pattaya City Police Colonel Anek Srathongyoo said on Monday.

“He explained that he’d been out for the night and had gotten into a fight with a friend, which was why he had those marks.

“It wasn’t until he was with us, and we showed him the evidence, starting with the footage of him dragging the suitcase, loading it on the back of his motorbike and riding away for around 10 to 20 minutes – and then returning without the suitcase – that he began to admit what had happened.”

Carman, who was born in Ballarat and has lived in Western Australia, claims self-defence, police say.

He is accused of picking up 17-year-old Thanchanok Donhomla for sex at about 3am on Thursday and taking her back to his unit in a multi-tower complex in Jomtien, a satellite area of Pattaya, about 150 kilometres south of Bangkok.

People who know him said he had been living in the tower for at least eight months.

The two are seen in CCTV footage holding hands in the foyer on their way up to the 15th floor at 3.34am. Nong Cake, as the victim was known to people close to her, was never seen leaving the complex.

She had travelled alone to Jomtien from Thailand’s north-east only a week earlier to meet friends, one of whom made a missing persons report on Friday when she failed to turn up.

“Normally, we would issue a missing-person alert. But in this case, something didn’t feel right to me,” Srathongyoo said.

“We’ve had cases before where a foreign suspect committed a crime and then rushed to catch a flight out of the country. So I told my team to check whether the missing-person report was genuine … and then decided to go back to where it all started with the CCTV footage [at the condo tower].”

Thanchanok may have already been dead for hours by the time one of Carman’s neighbours saw him eating breakfast at the complex. Cameras from his building capture him throughout the day doing mundane activities and speaking to people in the foyer.

“From around 3.30am until about 9pm, when he disposed of the body, he appeared to go about his daily routine as usual. We watched every single clip. He went downstairs to buy food, bought some groceries, did his laundry – everything seemed completely normal,” Srathongyoo alleged.

“Then we came to the part of the footage where he’s with the suitcase … but instead of leaving through the front entrance, he exited through the back door.”

CCTV footage shows him driving through the streets with the suitcase strapped to the back of his rented scooter, before returning without it.

The problem for investigators was that there was no camera vision for a two-kilometre stretch of road. So Srathongyoo sent out 10 investigators to comb the entire area of thick grass, finding a suitcase only a few hours after Carman had been detained at the airport on Friday.

Thanchanok’s body was found inside. Her clothes and possessions – a vape, a phone and a couple of receipts – were jammed into the spaces between her limbs, Srathongyoo said.

Srathongyoo said Carman eventually told investigators the girl had come at him with a knife, and he had strangled her, unintentionally killing her. Thai media reported that the pair argued over the equivalent of just $20, but Srathongyoo would not comment, as Thanchanok was not alive to tell her version.

Carman faces three charges: murder, concealment of a body, moving or destroying a body, and taking a minor for indecent acts.

In a video captured after his arrest, Carman addressed the victim’s family, saying he felt “bad” about the incident.

“I feel bad for what happened to your daughter. It was out of my control,” he said. “I know you’ll be very sad, upset, same, same me. It shouldn’t happen, and I hope you’re OK. I know you’re not, but I hope, and please tell other girls … to be careful.”

The victim’s father, Thongchai Donhomla, spoke of his grief outside a police station in Pattaya.

“I am deeply saddened. My daughter had no mother because we’ve been divorced since she was two years old,” he said in an interview with Thailand’s TMN Cable TV Pattaya.

“She was a good kid. Whenever she wanted anything, she would find a way herself, and she always helped me, too. She never bothered me.”

Carman lived for a time in an industrial corner of Rockingham, a coastal city in WA about 50 kilometres south of Perth. He had previously owned firearms, but WA court records show his licence was cancelled and firearms were confiscated by the state’s police commissioner. An appeal against the move was rejected in 2024.

In Thailand, a murder conviction can carry the death penalty or imprisonment of 15 to 20 years.