A 13-year-old boy found dead on a riverbank in Kawasaki who may have been the victim of bullying was killed with multiple sharp objects, investigative sources said Tuesday, noting the victim had sustained a deep stab wound to the neck.

A friend said Ryota Uemura had revealed that he had been abused by a group of older boys who may have tried to kill him.

The victim started to get involved with the group in November. He was worried about the escalating violence after he said he wanted to leave the group, the friend said. Uemura was quoted as saying that he wanted to go to school but was threatened into not attending. Uemura told the friend the violence against him began when he refused to shoplift after being ordered to do so. Other friends had witnessed Uemura, his face swollen, being accompanied by the group earlier this month.