Police delayed seizing the clothes of Sanjay Roy, an accused in the rape and murder of a trainee doctor in Kolkata's RG Kar Medical College, for two days though his role had been established on August 10, a day after the crime, CBI officials said on Wednesday.

Roy, a police volunteer, was arrested based on CCTV footage showing him entering the seminar hall of the college at 4.03 am on the day of the incident.

His role in the "commission of crime had already emerged" but Tala Police caused an "unnecessary delay of two days" in seizing his clothes and other belongings. It could have resulted in strong evidence against him, the officials said.

The central probe agency, which took over the case on August 14 following an order from the Calcutta High Court, has arrested former principal Sandip Ghosh and Tala Police station officer in-charge Abhijit Mondal in connection with the case. Both were found to be evasive in their responses, the officials alleged.

The officials also accused the two of "malafide intention" and said they initiated the investigation with an attempt to "vitiate the evidence" and "vital data related to this case."

The CBI is now probing criminal conspiracy, if any, between Roy, Ghosh and Mondal, they said.

The CBI will certify details of "each and every phone call" to verify versions with a view to explore "the possibility of criminal conspiracy hatched among main accused and co-accused persons, if any", an official said.

The agency will confront the two with the CCTV footage from the Tala police station, the scene of the crime, the medical college and also examine mobile data extracted from their phones to "unearth the conspiracy/nexus, if any and attempt to hush up the matter", he said.

Mondal and Ghosh allegedly facilitated the cremation in a hurried manner while the family members specifically demanded second autopsy, officials said.

Ghosh remained "wilfully absent" from the crime scene instead of closely monitoring the proceedings and taking necessary steps for its preservation and registration of FIR at the earliest, the CBI has alleged.

The agency is also investigating allegations against Roy and Mondal from the family members of the victim as well as from open sources, they said.

The trainee doctor was allegedly raped and murdered in the seminar hall of RG Kar Medical College and Hospital when she had gone to rest during her 'graveyard' shift in the early hours of August 9. Her body with severe injury marks was found inside the hall by a doctor who was on rounds.