A shooting at a high school near Nonthaburi, northwest of Bangkok, has left nine people dead, including teachers and students, according to Thai authorities.

Police said a student opened fire on the campus, killing three teachers and three students among the victims. Fifteen others were injured, with two reported to be in critical condition.

Investigators said the student is believed to have used a firearm taken from a family member. The reason for the attack has not yet been established and an inquiry is continuing.

Thai Prime Minister Anutin Charnvirakul expressed condolences to the families of those killed and instructed officials to provide all necessary assistance to the affected.

Police also said the student allegedly shot and killed his grandparents at home before the school attack, and later died by suicide. A similar incident was reported in southern Thailand in February, when a 17-year-old’s school shooting killed a teacher.