The CBI has filed a charge sheet against the prime accused, Sanjay Roy, in the rape and murder case of a trainee doctor at R G Kar Medical College and Hospital in Calcutta, officials said on Monday.

In its charge sheet filed before a special court in Calcutta, the Central Bureau of Investigation (CBI) said Roy, who was working as a civic volunteer with the local police, allegedly committed the crime on August 9 when the victim had gone to sleep in the hospital's seminar room during a break, they said.

The agency did not mention the charge of gang rape, indicating that Roy alone committed the crime, according to the officials.

Kolkata Police had arrested Roy on August 10 on the basis of CCTV footage which showed him entering the seminar room in the early hours of August 9, they said.

The CBI, which took over the investigation on August 14 following a Calcutta High Court order, also took his custody and conducted a detailed interrogation, followed by a polygraph test.

Roy had refused to give consent for the narco analysis, following which the CBI could not proceed with the test.

The CBI has also arrested Tala police station officer in-charge Abhijit Mondal and former principal of the medical college Sandip Ghosh in the case. Ghosh is also facing another CBI case pertaining to alleged corruption.

The body of the trainee doctor was found at 9.30 am on August 9 by her colleague who went looking for her before starting ward rounds. The Tala police station was informed about the "body of a lady lying in an unconscious state" and its team reached the spot around 10.30 am.

The junior doctor's killing sparked a protest by resident doctors across the country, demanding better security for them.

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