Ram Singh's lawyer, VK Anand, has questioned the police version that his client killed himself.
The police document also cites a confession that it says was made by Ram Singh.
Journalist Namita Bhandare feels "no doubt that Ram Singh's suicide is yet another lapse by police".
On Dec. 17, police say, they found Ram Singh on the bus and took him into custody.
Meanwhile, Punjab police on Wednesday night said the boxer's sparring partner, Ram Singh, had been arrested "for custodial interrogation".
K. Anand's initial statement that he believed the death of his client Ram Singh to have been a suicide.
The five accused have been named as Ram Singh, his brother Mukesh, Pawan Gupta, Vinay Sharma and Akshay Thakur.
The five accused are Ram Singh, his brother Mukeshand and their colleagues Pawan Gupta, Vinay Sharma and Akshay Thakur.
One of the suspects, Ram Singh, was found dead in his shared cell at Delhi's Tihar jail earlier this month.
Senior prison officials I spoke to say they are surprised to hear that Ram Singh was not put on suicide watch.
On Wednesday night, Punjab police arrested the boxer's sparring partner, Ram Singh.
The five defendants facing trial in the Saket fast-track court are Ram Singh and his brother Mukesh, Pawan Gupta, Vinay Sharma and Akshay Thakur.
BBC: Delhi rape case: Defendants face first witness evidence
The lawyer for Ram Singh, VK Anand, said he would ask to move the trial out of the capital because of the media attention.
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.
Ram Singh's family and lawyer alleged foul play in his death.
Most Indian news TV channels and websites are giving wall-to-wall coverage of the death of Delhi gang rape accused Ram Singh in the capital's Tihar jail.
Ram Singh, 33 years old, was found dead in his cell at Tihar Jail early Monday in what police and prison authorities said was suicide by hanging.
Ram Singh and the four other adult defendants - his brother Mukesh, Pawan Gupta, Vinay Sharma and Akshay Thakur - have been on trial in a fast-track court.
BBC: Delhi rape: Ram Singh's body released after post mortem
Mr. Anand, the lawyer representing Ram Singh, said he didn't dispute that his client made a statement to police but said it wasn't, under Indian law, admissible as evidence in court.
But some of his relatives say Ram Singh was a strong man who, even after the 2009 accident, did not quit driving because a rod had been inserted into the damaged arm.
Ram Singh, 34, also had a badly injured hand and would have been unable to orchestrate a suicide, his father said, speaking from outside his small home in a New Delhi slum.
The lawyer for Ram Singh, said he would file a petition arguing for the case to be transferred out of Delhi, fearing that the proceedings might be prejudiced because of the intense media interest.
Tihar jail officials said Ram Singh appeared to have hanged himself with an improvised rope made from a blanket at about 05:00 local time on Monday (23:30 GMT Sunday) while his three cellmates were asleep.
BBC: Delhi rape: Ram Singh's body released after post mortem
Ram Singh, who is accused of driving the bus on which the 23-year-old student was raped and fatally assaulted by a group of six men in December, hanged himself with his own clothes, said G.
His father, Mangelal Singh, also insists that Ram Singh would not have been able to orchestrate a suicide because he only had the effective use of one arm- Ram Singh had damaged his right arm in a driving accident in 2009.
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.
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