Odd job worker K Nagarajan, 32, was arrested on Dec 21 for alleged drug related offenses and was placed in the Dang Wangi district police headquarters lock-up.
He was found dead on Monday before his scheduled court appearance.
According to his uncle M Krishna Murthie, the family was unable to accept the police version of events.
"How could someone die by falling in the lock-up? How is that even possible?" he told a press conference, organised by Teratai state assemblyperson Jenice Lee today.
Krishna Murthie said that there were also mysterious wounds - a deep gash above the left eye, a coin sized bruise above the right eye brow and a wound on the right leg - found on Nagarajan.
He said that no explanation was provided by the authorities on how Nagarajan could have sustained those wounds.
Autopsy report not out
He also complained that the family was not informed of Nagarajan's arrest and that the police took several hours to send the body for an autopsy.
Nagarajan's mother M Nagamah, 58, said she wanted the police to explain, in detail, how her son had died.
"I don't want the same to happen to other people, this should stop," she said.
Lee said that the authorities owe the family a proper explanation to provide them with closure.
"It's been a week and the autopsy report is still not out. This is not fair to the family. They want to know why he went in the lock-up alive and came out dead.
"Even after the Kugan case, death in custody is still happening," she said.
She explained that the deceased had psychiatric problems and required medication and it was the police responsibility to see to his medical needs during his detention.
Lee added that DAP will provide legal aid and Sungkai state assemblyperson A Sivanesan will serve as their lawyer.
When contacted, Dang Wangi OCPD Zainuddin Ahmad said investigations are underway. He declined further comment.