The taekwondo instructor, who allegedly abused a five-year-old boy causing brain death, leaves a court in Uijeongbu, Gyeonggi Province, after being questioned, Sunday. Yonhap

Police are continuing their investigation into a taekwondo instructor who abused a five-year-old boy at his martial arts studio in Yangju, Gyeonggi Province, resulting in the child's brain death.

The instructor forced the boy into a rolled-up mattress in an upright position during a lesson on Friday. The five-year-old had to stay with his head down for at least 10 minutes, and by the time the instructor took him out of the mattress, the boy's heart had stopped beating, according to the police.

The instructor told police it was an accident that happened during an activity. However, police allege the instructor abused the child and discovered that he had attempted to delete parts of the security camera footage at the studio.

The instructor was detained Sunday, and the footage is under forensic analysis.

On social media, the boy's mother wrote, "I can't believe what happened to my son. His brain didn't function, and I saw all the blood vessels being ruptured on his face (when I arrived at the hospital). Doctors are administering medication to stimulate heart activity. But, the end could be near, and we are all here to stay next to him."

The instructor faced a new child abuse allegation Monday from another parent who said their child had been beaten at the studio. The police are also looking into the case.