Fredrick Muriithi Wanjiru, 30, was repeatedly beaten on the head with an object, rupturing the soft tissues.
The injuries inflicted caused internal bleeding in the brain tissues, a postmortem report shows.
His chest also bore extensive bruises suggesting that as he endured the merciless beating, he was being dragged on the ground, his chest down.
There were also cuts on the forehead, ranging between four and seven centimetres, the exercise showed.
The postmortem exercise was done on August 12.
Muriithi, alongside his younger brother Victor Mwangi, is among the four men lynched by a mob in Kitengela, mistaken for cattle rustlers on August 8.
The others were Mike George, 29, and Nicholas Musa, 28 who were cousins.
The report seen by the Star shows that the repeated blunt force beating inflicted trauma on the internal systems of his head.
Though the head had no raptures, the global subgaleal had extensive contusions, a medical term for bruises.
The nervous system also had extensive bleeding, with report recording that he suffered “global subdural and subarachnoid haemorrhage.”
"[There were] deep contusions, extending to the muscles, on lateral aspect of both upper limbs, lower limbs, posterior trunk [and] anterior trunk," the report reads.
Further, he also suffered broken right shoulder. His right leg below the knee also got fractures, the report showed.
On the day Muriithi and the three met their gruesome death, he alongside Mwangi, on a motorbike, left their house looking for a place to buy a chicken in Kisaju area. It was at 6pm.
There was a pending merrymaking to mark Mwangi's 26th birthday.
They stopped near a riverbank at Enkamulyat area, some 14km from Kitengela town.
Residents who saw motorbikes parked by the riverbank thought the owners were out to steal their cattle.
The residents raised the alarm and without listening to the young men's pleas, descended on them with spears and clubs.
The area which is commonly referred as Kitengela Reserve has had cases of cattle theft.
Close friends said the four had arrived in Kitengela in three motorcycles.
Upon their arrival, the four went to George's sister's newly built house where they were hosted.
(Edited by Bilha Makokha)
