A 28-year-old man is in custody after the shooting deaths of his father-in-law and a brother of his wife at a townhouse in Bang Kae district of Bangkok on Saturday morning.
The man was preparing to kill his mother-in-law when he was caught, said police.
The fatal shootout occurred inside the two-storey townhouse at the Ban Suk Samran housing estate in Lak Song area of Bang Kae, said Pol Capt Kittipong Bunset, deputy investigation officer at the Phetkasem police station, who was alerted around 7.45am.
On entering the house, police and rescue workers found the body of a man, identified later as Pongsathorn Seetapanya, 22, lying face-up on the first floor with a gunshot wound to the left temple. Two spent cartridges were found nearby.
At the back of the house near the wall, the body of another man, identified later as owner Thanin Seetapanya, 46, was found with a gunshot wound to the left temple. One spent cartridge was found nearby. Police also found two bullet holes in the door of a second-floor bedroom.
Prapassorn Sawetanai, 44, the wife of Thanin, told police that the gunman was her son-in-law Nathee Panthumsen, 28. He had threatened to kill her family members as they disapproved of his relationship with her daughter, she said.
Mr Nathee had first fallen in love with her daughter Thanyarat when they were in school, said Ms Prapassorn. However, he got involved with drugs and she asked her daughter to end the relationship.
The couple separated for a while but later got back together and had a baby boy. They moved to another house in Phetkasem Soi 110 and Nathee asked his wife not to contact her family, said the mother.
The dispute intensified and the man threatened to kill all of the members in her family, said Ms Prapassorn. She recorded his threatening messages and filed a complaint with police at the Phetkasem station on Aug 14.
The woman said her son-in-law did not work to support his family. Her daughter, who is now seven months pregnant, has had to go out to work.
Before the fatal shooting, Thanyarat contacted her mother via Line chat to warn her that Nathee was heading to the house to shoot everyone. While they were talking, Ms Prapassorn heard the sound of two gunshots. Her son, who had been sleeping on the first floor and did not lock the door of the house, was dead. Her husband, who was preparing to take a bath on the first floor, was then gunned down, said the woman.
She immediately locked her room on the second floor and the gunman went upstairs and fired shots at the door of the room. She ran to hide in the bathroom inside her room and phoned a friend to alert police.
The gunman was subsequently arrested at a pavilion at Wat Bang Muang, opposite the Phetsakem police station.
The arrest came after an officer noticed the man, wearing a face mask, who looked like the gunman based on the description police had been given.
Pol Snr Sgt Maj Maitree Khowiang, of Metropolitan Police Division 6, was walking toward his vehicle in the car park of the temple following a meeting with investigators handling the case at around 11am.
As the man was acting suspiciously, the officer approached him and talked with him briefly before asking him to remove his face mask. On seeing the face, the officer immediately arrested the suspect. A gun with eight rounds of bullets was seized from him.
During questioning, Nathee told police that he had waited at the pavilion as he expected his mother-in-law would be coming to the police station to give a statement about the shootings. When he saw her, he planned to shoot her.
Police are awaiting the result of a drug test on the suspect, said Pol Maj Gen Pong-anan Khlaikhueng, commander of Metropolitan Police Division 9.