A Lao national suspected of killing a 67-year-old woman and her 12-year-old granddaughter in their house in Samut Prakan was arrested in the border province of Mukdahan early on Monday morning.

The 22-year-old, identified only as Phu, was apprehended by immigration officials in front of a grocery store in Ban Na Po Noi, Muang district, while he was waiting to take a ferry across the Mekong River to Savannakhet in Laos, according to Thai media reports.

The man is accused of killing Mali Im-sawat, 67, and a 12-year-old girl, and seriously wounding a 9-year-old girl in their house in Soi Phet Hueng 23 in Samut Prakan's Phra Pradaeng district. The three victims were found with their throats slashed. A cat was also found dead with its throat cut.

Police launched an extensive manhunt for Mr Phu, who worked as a welder in the area, after he was seen leaving the house with blood stains on his clothes. Valuables were also missing from the house.

According to police, Mr Phu initially admitted to attacking the three victims after breaking into the house looking for money and gold jewellery. He thought the old woman would have a lot of money because she owned a grocery store. He took a knife from the house and cut the cat's throat when it hissed and growled. He attacked the woman as she walked in on him ransacking the room and then the girls.

He fled, taking a taxi motorcycle to a supermarket and then a taxi to Mo Chit bus terminal where he changed clothes and took a bus to Yasothon. There, he sold the gold before taking another bus to Mukdahan.

He was having a beer at the grocery store when approached by immigration officials and taken into custody. Mr Phu is to be handed over to police in Samut Prakan.