The body was found without clothes on Tuesday. It is suspected that the boy was stabbed with a screwdriver and the burn injuries were from a cigarette, they said.
A school dropout, the boy had fallen into bad company, a senior police officer said, adding that the child used to feel neglected.
The post-mortem report will confirm if the boy was sexual assaulted, the police said, adding that an initial probe suggests that the cause of death was a puncture wound to the neck.
More than 40 hours after the boy's body was found, investigators have not been able to determine the motive behind the brutal killing.
"We have questioned more than 25 people of all ages to determine the motive but so far we have not been able to draw a conclusion," the officer said.
All angles, including personal enmity with the boy's family, are being investigated, police said.