The man accused of murdering his wife and four in-laws in Uttaradit is also alleged to have swindled almost 30 million baht from a priest, the rector of a Catholic college in Chon Buri province.

Police looking into the origins of Teerapon Pinanon's wealth found that he is also accused of duping Chaiyo Kitsakul, 66, a priest he had known for several years, into lending him 29.86 million baht to help clear a case involving the sale of land in Phuket, Thai media reported on Wednesday.

The money was never repaid and Mr Teerapon did not own the land, according to a police complaint later filed by the priest.

Mr Teerapon, 37, was arrested in Ranong near the Myanmar border on Tuesday morning by a team of police and soldiers after fleeing from Uttaradit. 

Mr Teerapon's mother-in-law, Namphueng Kingkaew, and her two elder sisters, Nampha and Kanokwan Inthasit were shot dead on Sunday 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.

Teerapon Pinanon, after his capture in Ranong province on Tuesday. (Photo supplied)

The suspect was released from jail after serving time for drugs before he met his wife. Witnesses told police he told his wife he had made a lot of money investing in stocks.

Police investigators learned that Mr Teerapon had driven to see the 66-year-old priest at Darasamut School in Sri Racha district, Chon Buri, last year.

Mr Teerapon allegedly told the priest he had sold land he owned in Phuket, about 10 rai, for 35 million baht, but the money had been frozen by authorities because his relatives faced drug charges.

Mr Teerapon produced land documents in his name and persuaded Rector Chaiyo to lend him 29.3 million baht. When he was due to pay back the money, Mr Teerapon kept postponing payment and then disappeared.

Alarmed, the priest checked with the Phuket land office, which told him Mr Teerapon was not the owner of the land he claimed to have sold.

The priest then filed a complaint with Sri Racha police, accusing Mr Teerapon of swindling him out of the money, the reports said.

In Uttaradit on Wednesday, police made Mr Teerapon go through a re-enactment of his alleged cime.

Uttaradit police said the re-enactment was at the provincial police office instead of where the murders occurred because police feared the victims' angry relatives and friends would attack him.

Police told media the suspect shot dead his wife, her parents and two aunties in a dispute over assets he previously gave to them.

The suspect asked for the return of money and other assets from his wife’s family because she wanted to end her relationship with him.

The family refused and told him to take the matter to court if he wanted his assets back.

The police also confirmed that about 30 million baht worth of the suspect's assets were obtained with money obtained from the priest in Chon Buri.