India's interior minister has described as "a major lapse" the security in a prison where a man on trial for the gang rape and fatal beating of a woman aboard a New Delhi bus committed suicide.
Sushil Kumar Shinde made the comments on Monday hours after it emerged that Ram Singh, who was accused of driving the bus on which the 23-year-old student was raped by a group of six men in December, had hanged himself.
Singh, who used his own clothes to take his life, had been put on suicide watch at New Delhi's Tihar Jail, police said.
The minister said an inquiry was under way and would be headed by a magistrate.
Singh's death is raising further questions about a criminal justice system already being criticised for failing to protect the nation's women.
Singh's co-accused are facing the death penalty if convicted of the attack, which horrified Indians and set off nationwide protests.
A sixth accused is being tried and jailed separately because he is a juvenile.
Ram Singh's family and lawyer alleged foul play in his death.
"There were no circumstances which could have led to Ram Singh committing suicide. There was no mental stress. He was very happy," his lawyer VK Anand said.
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.
He he added that when he visited his son four days ago, he appeared fine and gave no hint of the despair that could have driven him to take his own life.
The 34-year-old also had a badly injured hand and would have been unable to commit a suicide, his father said, speaking from outside his small home in a New Delhi slum.
"Somebody has killed him," he said, insisting he would push for a top-level investigation into the death.
Speaking to Al Jazeera, Kavita Krishnan, a women's group advocate, said Singh's death in custody was disappointing for those who wanted to see due process take its full course.
“How could he die in a high-security jail? I think there are a lot of unanswered questions about the suicide," Krishnan said. "We are demanding a high-level judicial inquiry, an impartial judicial inquiry into the circumstances of the death.”
The trial of the five adult men began in a special fast-track court last month while the juvenile's trial began 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 16.
The woman was repeatedly raped and tortured with a metal bar. The couple were also severely beaten before being thrown onto the road.
The woman died of internal injuries in a Singapore hospital two weeks later.