The civil volunteer of the Kolkata Police, was arrested on August 10, a day after the victim's body was found in the state-run hospital's seminar hall.

The federal agency officer, quoting doctors from New Delhi's Central Forensic Science Laboratory (CFSL), said that Roy had an "animal-like instinct".

"The man showed no repentance and narrated the entire episode giving ever minute detail without details. It appeared that he had no remorse," the CBI official told news agency PTI.

Notably, several pornographic contents were also found in Roy's mobile phone, which was seized by the Kolkata Police before the probe was handed over to the CBI following the Calcutta High Court's order.

The pieces of evidence, both technical and scientific, "very much support" that the accused was present at the crime scene, the officer added.

Referring to the CCTV footage from the hospital, the officer said that Roy could be seen near the chest medicine department ward at around 11 am on August 8.

"Footage showed him entering the same building again at around 4 am on August 9. Certain technical and scientific evidence corroborated that," he said.

The officer, however, did not reveal anything about the findings of the DNA tests conducted on Roy and did not comment on the ongoing speculations of gangrape in this case.

Notably, officers of the central probe agency have visited Roy's residence in Bhowanipore, spoke to his family members, neighbours, and also to his colleagues in the Kolkata Police force.

WHAT DID AUTOPSY REPORT SAY

The postmortem examination of the deceased doctor had revealed that her death was primarily caused due to "manual strangulation associated with smothering", adding that her manner of death was "homicidal".

According to the autopsy report, the victim had 16 external injuries and nine internal injury marks. Doctors said that there was medical evidence of "forceful penetration/insertion in her genitalia -- the possibility of sexual assault".

The external injuries included wounds on her cheeks, lips, nose, neck, arms and knees, while injuries to her private parts were also mentioned in the report.

It was also reported that the deep injury found in her private parts was caused by "perverted sexuality" and "genital torture".