The man police say was the ringleader in a gang rape on a New Delhi bus in December that outraged India, hanged himself in his cell yesterday, prison authorities said, raising questions about jail monitoring and security.

Ram Singh made a noose from the mat he slept on and hanged himself just before dawn from a grille in the ceiling of the cell he shared with three others, prison spokesman Sunil Gupta said.

While there were CCTV cameras throughout Tihar, India’s highest-security prison, there were none in individual cells, Gupta said. Singh had been placed in the “ordinary security” category, so did not receive any special surveillance, he said.

Gupta could not say how long it would have taken Singh to make the noose or how he had managed to loop it through the grille, which was 2.3 metres above the floor. He may have stood on a plastic bucket, some Indian media reported.

Singh’s lawyer, VK Anand, said his client had been composed and calm when he spoke to him in court on Friday and “didn’t have any complaints”. Singh, who faced the death penalty if convicted of murder, had not been on suicide watch, Anand and Gupta 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,” Anand said. Anand had previously denied that his client was being maltreated in prison. He would not elaborate on the “torture”.

Federal Home (Interior) Minister Sushil Kumar Shinde called the incident a “major lapse” in security and said an inquiry had been launched.

Singh and five others, including a juvenile, were put on trial for the December 16 attack on the 23-year-old trainee physiotherapist. The assault triggered nationwide protests, a toughening of rape laws and an intense debate about rampant crime against women in India.

All six accused have pleaded not guilty to rape and murder, although police say they admitted involvement in the attack in statements taken after their arrest.

Singh’s father said he did not believe his son had committed suicide and suspected that he had been murdered in his cell. “He confessed about his mistake, then why would he commit suicide? He was prepared for any punishment the government would have given him,” Mange Lal Singh said.

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.