New Delhi (CNN) -- One of the men accused of gang raping and fatally beating a woman on a New Delhi bus was found dead in his jail cell Monday.
Police say Ram Singh hanged himself. But his lawyer and parents claim he was murdered.
"There was no reason for him to commit suicide. There is some foul play," Singh's attorney V.K. Anand said.
Singh was housed in New Delhi's Tihar Prison. Last year, the prison recorded 18 deaths, including two suicides, according to CNN affiliate CNN-IBN.
"Ram Singh did not kill himself. He was murdered," Singh's father, Mangi Lai, told the affiliate.
New Delhi's Chief Minister Sheila Dikshit said she has ordered an inquiry.
Authorties 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.
Most Indian women have stories of sexual harassment and abuse on public transportation or on the streets, according to the Indian Council on Global Relations.
CNN's Harmeet Shah Singh reported from New Delhi; Holly Yan wrote in Atlanta