* Ram Singh found hanged in Delhi cell
* "We can't rule out foul play," his lawyer says
* Singh ringleader of gang who raped, killed student -police
* Death a "major lapse" in prison security, minister says (Adds quote from Supreme Court lawyer)
By Annie Banerji and Satarupa Bhattacharjya
NEW DELHI, March 11 (Reuters) - 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 on Monday, 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 eight feet (2.3 metres) above the floor. He may have stood on a plastic bucket, some Indian media reported.
Singh's lawyer, V.K. 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 Dec. 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.
BLOODSTAINED SCHOOL BUS
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.
Police say the six attacked the woman and a male companion on the bus as the couple returned home after watching a movie. The woman was repeatedly raped and tortured with a metal bar. They were also severely beaten before being thrown onto a road.
The woman died of internal injuries in a Singapore hospital two weeks later.
Legal experts said Singh's death did not undermine the prosecution's case, which was largely based on DNA evidence and the testimony of the rape victim before she died and her friend.
"There will be no impact on the trial. This is a case of gang rape where all are held equally responsible, you can't say one is more to blame than the other," said Rebecca Mammen John, a Supreme Court lawyer who has worked on many rape cases.
The police report draws a picture of Ram Singh as the ringleader. On the night of Dec. 16, the men gathered at Singh's 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.
The victim's brother said he was "not very thrilled with the news that he killed himself because I wanted him to be hanged ... publicly.
"Him dying on his own terms seems unfair. But, oh well, one is down. Hopefully the rest will wait for their death sentence." (Additional reporting by John Chalmers, Anurag Kotoky, Matthias Williams, Frank Jack Daniel, Diksha Madhok and Nita Bhalla; Writing by Ross Colvin; Editing by Louise Ireland)