Subhash Chandra Mishra, a doctor with Ram Manohar Lohia Hospital in New Delhi, said he has "uncleared doubts" over his son's death and lost hope of getting any justice from the Jharkhand government.
"The death looked unnatural as there was blood on his face and cuts and bruises on his body. He had also suffered a fracture on his skull which does not happen normally in a case of drowning," he said in the letter.
He said the doctors seemed to be in a hurry to declare his death as natural. "The autopsy was done within 15 minutes and it declared him drowned," he said in the three-page letter sent to acting NHRC chief Shivraj V Patil and Chief Justice Y K Sabharwal.
Mishra said the friends of Abhishek have neither come to meet him nor attended the funeral. "There are reports that they were taken to the Chief Minister's residence. All these reports strengthen our doubts," he added.
"I do not have the resources to go and meet them in person. So I have sent letters," Mishra said.
Abhishek drowned on Friday while on a picnic at Dassam falls, about 40 km from Ranchi, with two other BIT students, including Union Railway Minister Lalu Prasad's daughter Ragini.
The post-mortem report said Abhishek died of drowning. His body was fished out and cremated in Ranchi on Sunday.