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.