There was internal bleeding and he had broken ribs, the report said.
The post-mortem was conducted at the Thrissur Medical College.
"Eleven persons have been arrested in the case. Some more persons, suspected to be involved in the incident, are under custody and being interrogated," Agali Deputy Superintendent of Police T K Subrahmanyan told PTI.
Madhu was beaten to death by a group of persons for allegedly stealing food articles from some shops in the forest-fringe Agali town.
Thrissur Range IG M R Ajith Kumar said cases under various IPC sections, including murder, and SC/ST Atrocities Prevention Act would be registered against the accused.
According to doctors, who performed the post-mortem, the death was due to the severe internal head injuries, he said.
Police would also explore the possibility of registering cases under the I.T Act against them for circulating his photo on social media, the IG said. .............................................
Body brought to tribal settlement ...................................................
After the post-mortem, the body was taken to Attappady, where people from various walks of life thronged to pay tributes.
Attappady is one of the most backward tribal settlements in the state.
Earlier in the day, Union Tribal Affairs minister Jual Oram said a report has been sought from the state chief secretary over the tribal man's death on Thursday.
"Our ministry has sought a report from the chief secretary about the incident and the action taken by the state government," the minister told a television channel.
The state government decided to provide a financial assistance of Rs 10 lakh to the bereaved family.
Chief Minister Pinarayi Vijayan directed the chief secretary to hand over the amount to Madhu's family at the earliest.
Madhu, who was said to be mentally unsound, was living in a cave in the forest for the past several months, his family said.
His mother, Mallika and sister Chandrika told television channels that a group of nearly 10-15 persons went to the forest and thrashed him.
"After being beaten up, Madhu was made to walk over four km with a heavy sack on his back," they alleged.
When he asked for water, he was mocked at, they alleged, quoting eye-witnesses.
They also alleged that some forest officials had allowed the local people to go to the area and attack Madhu.
The opposition Congress and BJP observed "hartal" at Mannarcaud taluk, under which Attappady falls.
Tribal activists staged a protest this morning, blocking traffic at Attappady.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)