A teenager who opened fire at his home and school in Thailand, killing eight people and himself, had taught himself how to use a gun via social media, police have said.

The boy's consumption of violent social media content, anxiety over his studies and family issues, and conflict with friends might have also fuelled his actions, authorities said.

Senior police official Atthapol Anusit told reporters on Sunday that they have questioned 17 witnesses so far while investigating the 14-year-old's motive.

Thailand's leader has vowed to introduce new gun laws as the country reels from its deadliest shooting in years.

Eight people have died after the shooting in Thailand's Nonthaburi province, which borders Bangkok.

Authorities say the student at the Debsirin Nonthaburi School - a local branch of a prestigious secondary school - had shot dead his grandparents at home using a handgun owned by his grandfather, before heading to school where he opened fire at teachers and students.

When asked by reporters if the act was premeditated, Atthapol said that was "probably" the case.

Evidence that police have gathered include the firearm used by the teen in the shooting, cartridge cases, his mobile phone and a knife found in the bag he had carried to school.

His computer at home indicates that he had accessed violent content on social media, police say.

Forensic officers previously said that the gunshots were "quite accurate", with several hitting "vital spots" on victims' bodies.

Atthapol said on Sunday that the police investigation did not find evidence that the teen had practised shooting or trained at a firearms range.

However, the teen's teacher had previously confiscated a BB gun that he had ordered online and brought to school, Atthapol said.

The teen had lived with his grandparents since he was two years old, as his parents had separated, authorities say.

The boy's mother, who is not named, told Reuters that the teen was a "bright, cheerful person" and "behaved normally".

His mother told Reuters that she had last seen the boy during a recent school holiday.

"We are truly devastated, because no one thought this would happen," she said.

The teen's father apologised for his actions, local media reported.

One of the school's teachers described the boy as a good student with good grades, while education authorities described him as an avid gamer.

Thai Prime Minister Anutin Charnvirakul said previously that his grandmother was a teacher who "might have been strict" with the teenager.

As investigations continue into the deadly shooting, many are mourning the victims with some laying flowers in front of the school.

On Sunday, dozens dressed in black gathered at a Buddhist temple in Nonthaburi province for the funeral of a 12-year-old girl, who was injured during the shooting and died two days later.

"She was a good kid. A kid who loved to do every activity," the girl's music teacher told AFP. "She loved to dance."