Police recovered the body of the five-year-old boy, who was kidnapped on Tuesday, at South Gobindarkhil village in Patiya municipality area of Chattogram district on Thursday.
The deceased was identified as Zaihan, son of Md Shahjahan of the village.
Police also arrested five people- Md Saiful, Shahanur, Niha, Nihan and Wasifa- for suspected involvement in the killing.
Ariful Islam Siddique, Additional Superintendent of (Satkania Circle) police, said Zaihan went missing while playing on road near his house on Tuesday noon.
Police said the child's mother found a letter threatening to kill the child if Tk 3 lakh in ransom is not paid.
A written complaint was lodged with the local police station.
During investigation, police, DB police and local people spotted the body of Zaihan, kept in a sack, from a water body in Purba Para, behind the house of the arrestees, in the morning.
The body was sent to Chattogram Medical College and Hospital morgue for autopsy.
Later, local people staged demonstration in front of Patiya Police Station protesting the murder.