A 14-year-old student shot and killed his grandparents at their home before opening fire at his school outside Bangkok on Friday, leaving at least seven people dead and more than a dozen wounded in Thailand's deadliest mass killing in nearly four years, according to Thai police and news agency reports.

The attack took place at Debsirin Nonthaburi School in the Bang Kruai district of Nonthaburi province, northwest of the Thai capital. The shooting began around 10 a.m. local time, NPR reported, citing Deputy Interior Minister Polapee Suwunchwee. Among the dead were teachers, school administrators and students, though casualty figures have been revised repeatedly through the day and officials have not released a final count.

Prime Minister Anutin Charnvirakul described the attack as premeditated and told reporters at the scene that the teenager had been under significant strain, ABC News reported. According to the prime minister, the gunman was carrying more than 30 rounds of ammunition when he turned the weapon on himself, and could have caused far greater loss of life.

Investigators say the boy used a handgun registered to his grandfather. He had been living with his grandparents rather than his parents, and authorities have not yet determined how he obtained the weapon, CNN reported. Relatives interviewed at the family home told NPR that he was quiet and academically capable, and said they were unaware of any conflict at home or at school.

An emergency worker who responded to the scene told Reuters that the gunman had shot himself in the head and was still alive when medical teams reached him.

The motive remains under investigation.

Debsirin Nonthaburi is a branch of a well-known secondary school network founded in Bangkok in 1885, with roughly 3,000 students enrolled at the campus.

Thailand has one of the highest rates of civilian gun ownership in Asia. The country's deadliest such attack occurred in October 2022, when a former police officer killed dozens of people, most of them children, at a childcare center in Nong Bua Lamphu province.