Mourners gathered outside the school, leaving flowers and cups of milk tea at the gate. A young couple placed white flowers before bowing their heads in prayer as others continued to arrive throughout the day.

Investigators said the teenager fired at least 26 rounds from a handgun belonging to his grandfather. Authorities said he had earlier killed his grandparents at their home before travelling to the school and opening fire.

The death of the 12-year-old brought the overall death toll to nine, including the suspected gunman and his grandparents. Twenty-three people were initially reported injured.

Police are continuing to investigate the shooting, including how the teenager gained access to the firearm.

Officers removed electronic devices and other items from the suspect's home, where a Nazi flag was seen hanging near the garage. Emergency responders also removed two bodies from the property.

Police said the teenager fired at least 26 rounds from a handgun belonging to his grandfather. Investigators believe he killed his grandparents before travelling to the school and carrying out the attack. The gunman later died from a self-inflicted gunshot wound.

Thai Prime Minister Anutin Charnvirakul visited the school following the shooting. He described the attack as "truly terrible" and said it should "never have happened".

Police are investigating the motive for the shooting and how the teenager gained access to the weapon.