A gunman shot and killed his grandparents at their home before killing six people at a school outside Bangkok on Friday, authorities in Thailand said.

Three teachers and three students were killed in the shooting at the Debsirin Nonthaburi School in the Bang Kruai district of Nonthaburi province north of Bangkok, the capital, police said. Fifteen others were injured, including two in critical condition, police said.

Police said the gunman, whom they identified as a student, had also died by suicide. They said in a statement that he had fired 26 rounds and that 34 other rounds of ammunition were found at the scene.

One 18-year-old student told Reuters that at first he thought he was hearing firecrackers.

“Then it started getting more frequent, like ‘bang, bang, bang’ and then it went quiet at one point,” the unnamed student said. “And the shooting started again.”

This is the second school shooting in Thailand this year.

One teacher was killed and a student was injured in February at a school in the southern district of Hat Yai.