Earlier, officials reported about eight dead and said that among the victims were also schoolchildren.
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Two teachers and three other school staff were killed at a secondary school in Nontaburi province, bordering Bangkok, according to the report.
The suspect, a 14-year-old boy, is also accused of previously killing his grandparents at home.
The report states that the attacker also died, and one police officer told the news agency AFP that he committed suicide.
Prime Minister Anutin Charnvirakul at the scene told journalists that “the police questioned his close friend, who said the suspect was stressed” because of studies.
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“His actions also show that he clearly planned” the attack, said Anutin Charnvirakul.
It is reported that the suspect used his grandfather’s pistol.
“The weapon was registered and kept at home, but somehow he gained access to it,” said the prime minister.
According to him, more than 30 other people were also injured, and about nine of them are in critical condition.
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Local media report that the shooter was a student of the mentioned school.
