Police on Wednesday scoured the area where a school janitor found the severed head of an 11-year-old retarded boy, seeking clues in what could be the work of a serial killer.

The head of Jun Hase was discovered by a custodian Tuesday in the western city of Kobe. In its mouth, according to police, was a note written in red ink asking, “Can you stop me?”

Police were investigating whether Hase’s death was linked to two assaults in March in his quiet residential neighborhood, located about 270 miles west of Tokyo. One girl was killed and another seriously injured in those attacks.

The girls attended a different elementary school than Hase, but all three victims were found in the same half-mile area.

Police have made no arrests in the earlier attacks and have refused to say whether any of the three assaults was sexual.

An autopsy was done Wednesday on a headless body, also discovered Tuesday, on a small hill 550 yards from where the head was found, police said. The body has not yet been confirmed as Hase’s, they said.