However, his family and lawyer said they suspected foul play.
Ram Singh, the main accused in India's most high-profile criminal case, was said to have killed himself yesterday in a cell he shared with three other inmates in New Delhi's Tihar jail.
Singh's lawyer, VK Anand, said his client had been composed and calm when he spoke to him in court on Friday.
Singh, who faced the death penalty if convicted of murder, had not been on suicide watch.
Mr Anand said: "I know he had a few complaints of jail authorities torturing him, but nothing that would make him take his own life. We can't rule out foul play. Nothing is adding up."
Singh's father also raised his doubts. Mange Lal Singh said: "He did not commit suicide, he was murdered."
Singh's mother Ram Bai cried uncontrollably as her husband helped her into a car in the Delhi slum where they live. "He left me," she said repeatedly.
Federal Home Interior Minister Sushil Kumar Shinde said it was a major lapse in security and that an inquiry had been launched.
Police have described Singh as the ringleader of five men and a juvenile on trial for the December 2012 attack on the 23-year-old trainee physiotherapist in Delhi.
The assault triggered nationwide protests, a toughening of rape laws and an intense debate about crime against women in India.
All six accused pled not guilty to rape and murder.
Supreme Court lawyer Ravi Kant said Singh's death could have an impact on his co-accused.
He said: "The others accused in this case will take the opportunity to say that Ram Singh was the main conspirator and they were forced into it and as a result they can get lighter punishments."
A former director of Tihar jail, Kiran Bedi, said Singh should have been kept isolated from the main prison population.
She said: "High-security prisoners cannot be part of the community. They must be separate because the other prisoners will not accept them."
The trial of the five adult men started last month and the juvenile's trial began last week. Singh's brother Mukesh Singh, gym assistant Vinay Sharma, bus cleaner Akshay Kumar Singh and fruit vendor Pawan Kumar are the other men on trial.
Under Indian law, the juvenile cannot be named.
Authorities barred reporting on the trial, which resumed in a fast-track court yesterday.
Police allege the six attacked the woman and a male companion on the bus. The woman was repeatedly raped and tortured. The couple were also beaten before being thrown on to a road.
The woman died of internal injuries in a Singapore hospital two weeks later.
The report used to charge the accused draws a picture of Ram Singh as the ringleader.
It states the accused gathered at his house. He came up with the plan of taking the bus out to look for a victim to rape, it said.
The police say they found him sitting in the blood-stained school bus, wearing a bloodied T-shirt, after the crime. A DNA test revealed the blood belonged to the rape victim, the report said.
The physiotherapist's brother said of Singh's death that he was "not thrilled as I wanted him to be hanged ... publicly."