Investigators have drawn this conclusion after carefully examining all the available security camera footage of the Phuket City assassination in traffic on Friday.
Previously, police had presumed the single shot that hit Khun Jintana, 42, in the head was fired from another car through the driver's window of her white Chevrolet.
Now detectives say that Khun Jintana was tailed by motorcycle not just from the Hilton Phuket Arcadia Resort and Spa in Karon to Chalong Circle, but all the way north along Chao Fa Road West to the t-junction with Kwang Road, in southern Phuket City.
When her Chevrolet pulled up at the lights at 8.08pm, the motorcycle moved up alongside. The pillion passenger fired a single shot through the driver's window, police said.
The motorcycle then turned around and headed back down Chao Fa West Road, police said.
All available security cameras, both in use by Phuket traffic authorities and by private businesses, had been looked at in making the judgement about the motorcycle, police said.
Detectives still believe that Khun Jintana, mother of two, was shot with a thai pradit homemade weapon.
The handgun fires a single shot that scatters like a shotgun. Khun Jintana took the full force of the blast as she sat at the intersection, speaking on her mobile telephone.
After the shot, her Chevrolet jumped forward and was halted when it flattened a lamp post.
Police say the motorcycle type and brand remains unidentified. The rider was wearing a safety helmet but the man on the back was bareheaded.
Khun Jintana's funeral began at Tha Krang Church in Phuket City at the weekend and continues this week. She had daughters aged six and 13.
In a Facebook tribute today, Khun Jintana's husband Winyoo said: ''Sleep well, my dear one. You are in the arms of the Father who loves us. You are in Heaven, our eternal home.
''I will be strong with the Lord, and I will make a good father and mother to our children. Know that I love you and I miss you so much.''