Sawmill worker P Babu, 28, was found dead in the Jempol police lock up at about 4am after he surrendered himself in connection with a robbery case.
When contacted, Jempol MIC Youth chief M Palani said the first post-mortem report classified the death as suicide.
The deceased had apparently used his shirt to hang himself.
"But this is unofficial. We were told that we would get the black and white report within seven days," he added.
'Strong willed person'
Palani said the family have rejected the findings because "Babu was a strong willed person and would not have this decision (to commit suicide)".
"We are also questioning the fact that he hung himself using his shirt. Shouldn't he be wearing a police issued t-shirt (for detainees)?" he asked.
Yesterday, Palani filed a police report calling for a thorough investigation into Babu's death.
Both MIC and PKR politicians have cried foul over the incident and offered to help the family secure a second post-term.
For years, activists and politicians have been complaining about the high number of fatalities involving Indian Malaysian police detainees.
In January 2008, a police constable was charged with causing hurt to extract a confession from A Kugan, 22, who died in police custody in Subang Jaya.
In July, P Gunasegaran, 31, was found dead in the Sentul police station. An inquest is currently underway.
