New Delhi, India (CNN) -- Authorities have completed an autopsy of Ram Singh, one of the men accused of gang raping and fatally beating a woman on a New Delhi bus.
Police have said Singh hanged himself in jail cell in Tihar Prison on Monday. His lawyer and parents claim he was murdered.
It is unclear when the results of the autopsy will be released.
"Ram Singh did not kill himself. He was murdered," Singh's father, Mangi Lai, told CNN affiliate CNN-IBN on Monday.
Singh's death in custody sparked harsh criticism of India's prison system.
"If prisons are to be considered rational institutions, rather than horrible holding pens, then prisoners' rights should be of utmost importance, and their protection the state's strongly held commitment," wrote The Indian Express newspaper in an editorial Tuesday.
According to the country's National Crime Records Bureau (NCRB), India had 1,382 jails in 2011 with a total capacity to hold 332,782 inmates.
However, the occupancy rate in those jails was around 112% that year, data posted on the bureau website show.
Last year, Tihar Prison recorded 18 deaths, including two suicides, according to CNN-IBN.
Authorities say Singh was the driver of the bus aboard which several men brutally raped a 23-year-old woman on December 16.
The gang rape, and the woman's subsequent death from injuries, prompted angry protests over the country's treatment of women and handling of sexual attacks.
Police charged Singh and four other men with murder, rape and kidnapping.
Their trial began in February. If convicted, the men could face the death penalty.
A sixth suspect is 17 and will be tried separately in juvenile court.
The December rape gripped India and led to calls for stricter laws on sexual assault and changes in cultural attitudes toward women.
Many Indian women have stories of sexual harassment and abuse on public transportation or on the streets, according to the Indian Council on Global Relations.