Police Colonel Thadsakorn Konthong said that the gun and bullets used in the attack belonged to the teen’s grandfather, with an initial inspection finding signs that the firearm had been searched for inside the grandfather’s bedroom.
International and local media have reported that the gun was legally registered and had a permit.
Thailand has one of the highest rates of civilian gun ownership in South East Asia, with around 10.3 million firearms in civilian possession, around four million of which are believed to be owned illegally.
The country has strict gun laws, which punish the illegal possession of arms with up to 10 years in jail, but enforcement of these laws has been an issue.
During his time as interior minister, Anutin introduced a number of gun control laws, including a short-term ban on new gun licenses and import restrictions.
Speaking on Friday, the prime minister said the tragedy “shouldn’t have happened”.
“I don’t just feel sorry for the victims but I feel sorry that this happened. How can this happen in our country,” Anutin added.