Ram Singh, 33, was found hanged to death in his small cell. It was unclear if he was hanged with his own clothing or a bed sheet. He was found hanging in his New Delhi cell about 5:30 a.m. Other inmates were also in the small cell.
Singh and five other men are accused of picking up a man and woman who were going home after seeing a movie on December 16. The man and woman boarded a bus Singh was driving.
The suspects then beat the man with a metal bar, raped and beat the woman causing severe injuries, then dumped both of them naked on the street. The woman died from her injuries two weeks later.
The suspects are facing the death penalty. The crime shocked the world and set off national protests.
Singh was held at one of India's best-known prisons. The death underscores concerns about India's criminal justice system.
"There were no circumstances which could have led to Ram Singh committing suicide. There was no mental stress. He was very happy (about the trial's course)," his lawyer V.K. Anand said.
The government had ordered a magistrate's inquiry and would take action after it received the report, he said.
In 2011, 68 inmates in India killed themselves and another eight were killed by fellow inmates, according to India's National Crime Records Bureau. Tihar Jail is badly overcrowded and its 12,000 prisoners are nearly twice as many as it was designed to hold. Bedi said that despite that, the treatment of inmates has improved over the past two decades as the jail became more transparent, with volunteers constantly coming in and prisoners better educated about their rights.
Lawyers for the defendants had previously accused police of beating confessions out of the men.
Ram Singh's father, Mangelal Singh, said his son had been raped in prison by other inmates and had been repeatedly threatened by inmates and guards. Nevertheless, he said he visited his son four days ago and the man appeared fine and gave no hint of any despair that could drive him to take his own life.
Ram Singh also had a badly injured hand and would have been unable to hang himself, his father said, speaking from outside his small home in a New Delhi slum.
"Somebody has killed him," he said, saying he would push for a top-level investigation from India's Central Bureau of Investigation into the death.
Vivek Sharma, a lawyer representing another defendant, said he planned to ask the court to provide greater protection for his client.
"In a high-security jail, an occurrence of this kind is highly condemnable. It raises the serious issue of security of the accused persons in the jail," he said.
"My clients don't feel safe in Tihar Jail," said another defense lawyer, A.P. Singh.
Singh's death comes as the trial was deep underway. The four surviving defendants were briefly produced in court Monday. A fifth defendant, a juvenile, is being tried separately.
K.T.S. Tulsi, a former top lawyer in the office of the solicitor general of India, said the suicide should have no impact on the trial, which is being held in a closed courtroom under a gag order that prevents news organizations from publishing details of the proceedings.
The Associated Press contributed to this report.