But Mr. Singh’s family and his lawyer said that he would not have been able to tie a noose, because his right arm had been seriously damaged in a bus accident. In addition, they said, he shared his cell in the Tihar prison complex with several inmates, making it difficult to believe he could have hanged himself without being noticed.

“I suspect there is foul play,” the lawyer, V. K. Anand, said. “There were no circumstances for committing suicide. His mental state was stable, the trial was going well, he was meeting with his family. I can’t understand why he would commit suicide.”

Some family members said Mr. Singh had been abused in jail.

“It is not suicide; he has been hanged by the police,” his father, Mangilal, said in an interview. He said his son had told him on Friday that the police were beating him in jail and that he was being pressed to change his lawyer.

Earlier Monday, the father told the television channel NewsX that Ram Singh had said other inmates raped him.

Mr. Singh’s brother Mukesh is one of four other men accused in the case, which is being tried in a “fast track” court in South Delhi set up for sexual assault cases. The creation of such courts was a direct result of the uproar over the fatal rape in December, as thousands of people across India, long angered by selective law enforcement and endemic corruption, vented their outrage over failures in preventing and prosecuting crimes against women.