His arrest came amid reports of him flaunting unsourced wealth, and that he may have been involved with a gang of drug traffickers and have given some of their money to members of his wife's family, who it was rumoured declined to return it.
Teerapon Pinamon, named in a court arrest warrant, was captured near the Kra Buri district checkpoint around 8.30am by a combined team of police and soldiers, Pol Col Thapset Khuenkaew, chief investigator of Uttaradit province, said. Kra Buri is the northernmost district of Ranong and adjoins Myanmar.
The suspect was being held in custody at the Ranong provincial police office.
Mr Teerapon's mother-in-law, Namphueng Kingkaew, and her two elder sisters, Nampha and Kanokwan Inthasit were shot dead at a house in Phichai district in Uttaradit and his wife Kanyarat Kingkaew and his father-in-law Wirat Kingkaew were killed shortly after at another house nearby.
Two other men were also wounded at the second house.
The murders were reported to police around 5.45pm on Sunday (Jan 13).
The suspect left the scene in a Toyota Camry sedan with Phitsanulok licence plates that was found left on Monday at a hotel in Muang district of Phitsanulok province.
Police said he had borrowed a motorcycle from a hotel maid and had not returned. They had believed he was trying to make his way back to his home province of Ranong, where he was caught on Tuesday morning.
Local residents earlier told police Teerapon was a jealous man and had often quarrelled with his wife. They had an argument on Sunday morning.
Mr Teerapon was released from jail after serving time for drugs before he met his wife. Witnesses said he told his wife he had made a lot of money investing in stocks.
News of the suspect's arrest did little to quell the fears of his slain wife's relatives.
Pratchaya Sertluecha, chief of Phichai district, said Kayarat's relatives in tambon Phayaman were in fear of their lives. They said Mr Teerapon had previously threatened he would kill all his wife's family.
They now feared the suspect might send friends or members of his network to take their lives.
The district chief said he had told them not to be afraid as police would protect them. Other local officials also told them to immediately contact district authorities or police if any stranger came to their community, Mr Pratchaya said.
“Officers and officials from relevant agencies had earlier tried to investigate his suspected unusual wealth. We asked how he could have earned so much money he could buy more than 10 cars, build a teak wood house worth over 3 million baht for his father-in-law and have shared a huge sum of money with his wife.
"What type of business was he doing?
"His claims that he invested in stocks were not convincing. We believe he amassed his wealth from the trade in illicit drugs.
"The man may not be a drug trafficker, but a person who kept the money for a drug network. He may have given some of the money to his wife and her relatives to look after. When the time came to return the money, the relatives may not have wanted to.
"This is one possible reason he was driven to kill five people,’’ the Phichai district chief said.
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