Thailand teen shooter once brought BB gun to school: police

People leave flowers outside Debsirin Nonthaburi secondary school the day after a 14-year old student shot dead 5 school staff, after also killing his grandparents at home. [AFP]

A 14-year-old boy who shot dead the grandparents who raised him before killing six others at his school in Thailand had watched violent videos online and once brought a BB gun to school, police said Sunday.

The teenage assailant shot his grandparents, using his grandfather's handgun, at their home north of Bangkok on Friday before going on a killing spree at his secondary school and ultimately turning the gun on himself, according to authorities.

A 12-year-old girl wounded in the school shooting died on Saturday, and five teachers and school staff were killed on Friday, police said.

Investigators interviewed 17 witnesses and were examining the boy's computer and phone to determine the "real cause of the shooting", Atthapol Anusit, a deputy police commander for the region surrounding the capital, told reporters on Sunday.

"We looked into his PC and found history that he watched violent videos on social media," Atthapol said.

"From questioning the kids, we know that he brought a BB gun to school last year but a teacher seized it."

Witnesses told police the boy had been interested in guns for a year or two, Atthapol said, adding that he had learned how to use one by watching content on social media.

The child had lived with his grandparents since he was two years old after his parents had separated, he added.

The boy's father apologised "to society for what my son did" on Friday, according to local media.

Prime Minister Anutin Charnvirakul has said the boy experienced school-related stress, and his dead grandmother was a former teacher who had been "quite strict on him about studying".

Thailand has one of the highest rates of gun ownership in the region, with around 10 million firearms estimated to be in circulation -- roughly one for every seven inhabitants.

Since Friday's mass shooting, officials have promised stricter gun laws, but previous pledges have failed to prevent repeated shootings.