One of the five men accused in the gang rape and fatal beating of a woman in New Delhi was found dead in his cell on Monday, police said.

Ram Singh, who was allegedly driving the private bus on which the 23-year-old medical student was raped in December, was found hanged with his own clothes at Delhi’s main prison, Tihar Jail, at about 5:30 am, police officials said.

Reports from Tihar Jail spokesman Sunil Gupta and Delhi’s assistant commissioner of police Rajinder Singh claimed that Singh “used the sleeping mat to make a noose (and) string from his trousers as a rope to hang himself” while his cellmates were sleeping.

Singh, 33, was under suicide watch at the jail where he and four other men were being held during their trial at a special court, according to the Wall Street Journal.

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

However, the defendant’s lawyer and family alleged foul play in his death, raising underlying questions on India’s criminal justice system.

“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.

Singh also had a badly injured hand and would have been unable to hang himself, his father told The Associated Press.

The five men face the death penalty if convicted of the rape and murder of the woman, a case that has sparked public outrage and widespread protests. The defendants have pleaded not guilty. A sixth suspect is being tried separately in juvenile court. Both cases continue to hear evidence.