Yasuhiko Kobayashi's 15-year-old son had skipped judo practice.

According to Kobayashi, the boy's teacher was furious and stood waiting for him at the gates of his junior high school in Yokohama. The teacher forced the boy into the gym and made him grapple one on one. The former All Japan judo champion choked the boy until he lost consciousness.

When the boy came to, the teacher choked him again until he went limp, and threw him to the floor with such force that he suffered severe internal bleeding in his brain, an injury known as an acute subdural hematoma.