Manan Vatsyayana / AFP - Getty Images

Media representatives surround an ambulance as it leaves the main entrance of Tihar Jail in New Delhi on Monday.

NEW DELHI -- The alleged ringleader in the gang-rape and death of a young Indian woman in December hanged himself in jail on Monday, officials said, a dramatic twist in a case that has provoked outrage across India.

Ram Singh's lawyer said his client had been composed and calm when he spoke to him on Friday and that there were other inmates in his cell in New Delhi's Tihar jail, raising questions about whether it was a suicide and how it could have gone unnoticed by staff in India's highest security prison.

The lawyer and a prison official said Singh had not been on suicide watch.

Police have described Singh as the ringleader of five men and a juvenile on trial for the December 16 attack on the 23-year-old trainee physiotherapist on a bus in the Indian capital. Singh was driving the bus.

All six accused have pleaded not guilty to rape and murder.

The assault triggered nationwide protests, a toughening of rape laws and an intense debate about rampant crime against women in India.

Tihar prison authorities ordered a magisterial inquiry after Singh was found hanging in his cell at around 5 a.m. (7 p.m. ET Sunday), prison spokesman Sunil Gupta said.

Singh's lawyer, V.K. Anand, told Reuters that his client did not appear to be distressed when he spoke to him on Friday.

"I believe he was satisfied with the way the trial was proceeding because we had a very strong case against the prosecution's claims," he said.

"This is not suicide, this is something else. 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," he said.

Manish Swarup / AP

The mother of Ram Singh cries as she speaks to journalists inside the family's home in New Delhi on Monday.

Anand has previously always denied that his client was being maltreated in prison. He did not elaborate on the "torture".

Singh had been kept in a cell with other inmates, he said.

A former director of the jail, Kiran Bedi, said Singh should have been kept isolated from the main prison population.

The trial of the five adult men started 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.

Under Indian law, the juvenile cannot be named.

The attack generated headlines around the world, but the case has since largely disappeared from public view, in large part because authorities have barred reporting on the trial, which was due to resume in a fast-track court on Monday.

Police allege the six attacked the woman and a 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 a road.

The woman died of internal injuries in a Singapore hospital two weeks later.

The police report used to charge the accused draws a picture of Ram Singh as the ringleader. On the night of December 16, the accused gathered at his house for dinner, where he came up with the plan of taking the bus out to look for a victim to rape, the report said.

The police say they found him sitting in the blood-stained school bus, wearing a bloodied T-shirt, the morning after the crime. A DNA test revealed that the blood belonged to the rape victim, the report said.

Related:

5 accused men plead not guilty in India gang rape

India gang-rape victim's father: Hang the 'monsters' responsible

India gang-rape victim dies in hospital; case focuses attention on sexual violence

This story was originally published on Sun Mar 10, 2013 11:22 PM EDT