Police arrested a 42-year-old high school teacher Friday after he broke into Tokyo Metropolitan Mita Senior High School and placed an explosive on the principal’s desk.

The explosive went off at around 11:40 a.m. while a metropolitan police bomb disposal unit was conducting a freeze-disposal. No one was injured, police said.

The suspect, Kotaro Matsumura, a mathematics teacher at Tokyo Metropolitan Hiroo Senior High School, told police that he had been planning revenge since he had been discriminated against by his colleagues.

According to police, Matsumura sprayed tear gas on a Mita high school official around 9:50 a.m. near the principal’s room when the official asked who he was. The official’s left eye was injured and he was taken to a hospital.

After he was arrested by police officers called to the scene, Matsumura reportedly made unintelligible remarks but in the process said he placed a bomb in the principal’s room.

Police then found the explosive in a small paper box. It went off just as the policemen were trying to dispose of it. Matsumura told police that he used powder from firecrackers and set the device to explode upon vibration.

The Mita school was almost empty at the time of the blast since students and teachers were attending a graduation ceremony at a public hall in Shinagawa Ward, police said.

Hiroo High School is located some 2.5 km from the Mita school, and Matsumura is believed to have driven to the nearby school after working in the morning at Hiroo, which was also holding a graduation ceremony.

Matsumura has told investigators that he was being subjected to discriminatory treatment and that he wanted to hurt the principal and other teachers at Mita “because I heard there are others who are doing the same thing,” police quoted him as saying.

Matsumura has reportedly said he chose Mita as the target because he would have been too easily figured out if he pulled the caper at Hiroo. The principal at Mita serves as chairman of a national association of high school principals.

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