A 6-year-old boy who was found dead on Thursday on the outskirts of Kobe, western Japan, appears to have died of shock caused by injuries.

The body of Hosaka Nao was found in a suitcase in a grassy area near his home.

Police say he had bruises over much of his back and the nature of the injuries suggest he was abused. The time of death appears to be approximately Monday.

Police arrested the boy's 34-year-old mother, Hosaka Saki, and her three siblings who live in the same house on suspicion of confining and assaulting Nao's grandmother, who is 57. She also lives in the same house.

They say the four suspects struck her on the back with what is believed to be a steel pipe around Monday, the same day the boy is thought to have died.

She fled from the house when the suspects were away, and was rescued on a street late Tuesday night.

Police are investigating whether the two cases are related.