Police said a man on trial for the gang rape and fatal beating of a woman aboard a New Delhi bus committed suicide in an Indian jail Monday, but his lawyer and family allege he was killed.

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, was under suicide watch at New Delhi's Tihar Jail when he hanged himself with his own clothes at about 5:30 a.m., police officials said. His death is raising further questions about a criminal justice system already being criticized for failing to protect the nation's women.

Singh, 33, had been among five defendants facing the death penalty if convicted of the attack, which horrified Indians and set off national protests. A sixth accused is being tried and jailed separately because he is a juvenile.

India's deputy home minister, R.P.N. Singh, said an inquiry had been ordered into the suicide, according to the Press Trust of India.

"The inquiry is being conducted and it would be premature to make any statement about the details of the incident," said Vimla Mehra, the director general of the jail.

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 V.K. Anand said. Lawyers for the defendants had previously accused police of beating confessions out of the men.

Indian jails have a reputation for overcrowding, poor management and brutal treatment of inmates.

'Somebody has killed him,' father says

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. Nevertheless, he said he visited his son four days ago and the man appeared fine and gave no hint of any despair that could drive him to take his own life.

Mangelal Singh, the father of Ram Singh, speaks to journalists as his mother weeps at the family's home in New Delhi on Monday. Police confirmed that Ram Singh committed suicide in an Indian jail, but his lawyer and family allege he was killed. (Manish Swarup/Associated Press)

Ram Singh also had a badly injured hand and would have been unable to hang himself, 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.

Mangelal Singh said he feared for the safety of another son who is also on trial in the rape case.

The defendants were being housed in separate buildings on the jail grounds and were all under suicide watch, a jail official said on condition of anonymity because he was not authorized to speak publicly on the matter.

Bus attack sparked protests

The rape victim and a male friend were attacked after boarding the bus Dec. 16 as they tried to return home after watching a movie, police say. The six men, the only occupants of the private bus, beat the man with a metal bar, raped the woman and used the bar to inflict massive internal injuries to her, police say. The victims were dumped naked on the roadside, and the woman died from her injuries two weeks later in a Singapore hospital.

The brutal attack set off nationwide protests about India's treatment of women and spurred the government to hurry through a new package of laws to protect them.

Singh's death comes as the trial was deep underway, with another hearing scheduled for Monday. The four surviving defendants were produced in court, but left after a short time because of an attorney's strike.

Vivek Sharma, a lawyer representing another defendant, said he planned to ask the court to provide greater protection for his client.

"In a high-security jail, an occurrence of this kind is highly condemnable. It raises the serious issue of security of the accused persons in the jail," he said.

"My clients don't feel safe in Tihar jail," said another defence lawyer, A.P. Singh.

Closed court for trial

K.T.S. Tulsi, a former top lawyer in the office of the solicitor general of India, said the suicide should have no impact on the trial, which is being held in a closed courtroom under a gag order that prevents news organizations from publishing details of the proceedings.

He said the death highlighted how important it is for society not to demonize people who have been accused but not convicted of crimes.

"It is so unfortunate that the media goes on to presume that they are guilty and goes on to condemn them and demonize them to an extent that it makes the life of these people not worth living," he said.

In 2011, 68 inmates in India killed themselves and another eight were killed by fellow inmates, according to India's National Crime Records Bureau.

Tihar Jail is badly overcrowded with nearly twice as many prisoners as it was designed to hold. Jail authorities have been working to soften its reputation in part by selling TJ's cookies baked by the inmates to the public.