The driver of the bus in which a young Indian woman was gang-raped and fatally injured three months ago hanged himself in New Delhi's Tihar jail on Monday, police said.
Ram Singh was the main accused of five men and a juvenile on trial for the attack on the 23-year-old trainee physiotherapist in the Indian capital.
The assault triggered nation-wide protests and an intense debate about rampant crime against women in India.
A senior police official said Singh had committed suicide in his cell early on Monday.
The CNN-IBN news channel said Singh hanged himself with his own clothes.
Tihar jail is India's highest security prison and officials there are likely to face tough questions about how such an incident could have occurred.
The 20-year-old brother of the victim said Singh knew he was going to die anyway because of the strong prosecution case against the attackers. He said the man's apparent suicide seemed unfair because he was dying on his own terms rather than awaiting a possible death sentence.
The trial of the five adult men began in a special fast-track court last month while that of the juvenile began separately last week. Ram 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.
The five men have pleaded not guilty to rape and murder.
Police allege the six attacked the woman and her male companion on the bus as the couple returned home after watching a movie on December 16th. The woman was repeatedly raped and tortured with a metal bar. The couple were also severely beaten before being thrown onto a road.
The woman died of internal injuries in a Singapore hospital two weeks later.
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