The shooting took place during a gold shop robbery at a Robinson shopping mall in Lopburi, Thailand, 160km from Bangkok, at around 9pm on January 9.
The masked man shot and killed a security guard, a young woman working at a gold shop, and then pointed a gun at a boy being taken away by his mother.
At least five others - including a tourist - were seriously injured and were taken to the hospital.
Police believe the masked gunman was a person trained in guns.
Mail quoted police colonel Kritsana Pattanacharoen as saying: "We believe the robber had been trained in the use of weapons before. The gestures he made when shooting showed that he was not someone who had never used guns before. The gun he used was a specialized weapon and it was not easy to get."
Kritsana said police had known the gunman's identity and were searching for the suspect.
The surveillance camera showed the gunman getting on the motorbike and fleeing before the police arrived at the scene.
