Khun Jintana, mother of two, was slain with a single bullet to the head when her white Chevrolet started off again from traffic lights at the Chao Fa West Road-Kwang Road intersection in Phuket City at 8.08pm.
The shot came from the car next door, a light-colored Toyota Vios, which sped off. Khun Jintana's vehicle came to a halt soon afer, when it knocked over a light post.
Investigators are looking at security camera footage that indicates she may have been tailed by a person on a motorcycle after she left the Hilton Phuket Arcadia Resort and Spa in Karon that evening.
The vehicle that contained her killer joined the traffic at Chalong Circle, and followed Khun Jintana, aged 42, north towards Phuket City.
A single fatal shot was fired as she talked on her mobile telephone.
More information is expected to be released by police this week. Detectives have been concerned that having too much detail in the media could alert her killer or killers.