A 25-member special team of the Central Bureau of Investigation led by an additional director on Wednesday began its investigation into the alleged rape and murder of a doctor at a state-run hospital in West Bengal as the agency took custody of the key accused, officials said.

A source in the CBI said the agency has filed an FIR under various sections of the Bharatiya Nyaya Sanhita in New Delhi in connection with the alleged rape and murder of the doctor.

The probe team primarily drawn from special crime unit of the agency and also its local unit immediately swung into action after reaching Kolkata this morning, taking possession of all the documents from Kolkata Police on the probe done by it so far, the officials said.

The team that also includes a joint director besides medical and forensic experts, and officers having extensive experience in special crime cases visited the RG Kar Medical College and Hospital's seminar hall, where the body of the trainee doctor was found on August 9, they said.

The Kolkata Police handed over key suspect Sanjoy Roy to the CBI at the CGO Complex in Kolkata after his medical tests were conducted at the state-run SSKM Hospital.

The CBI team will camp in Kolkata till the case is solved, the officials said.

Three groups of CBI officers have been formed to investigate the matter, an official of the central agency said.

"One group will visit the RG Kar Medical College and Hospital, and talk to the witnesses and the doctors who were on duty on that night. Another team will take the arrested civic volunteer to a local court after medical tests and plea for his custody, while another will coordinate with the Kolkata Police sleuths who were conducting the investigation," the official told PTI.

The Calcutta high court on Tuesday ordered the transfer of the probe from the Kolkata police to the central agency and directed the police to hand over the case diary to the CBI by the evening and all other documents by 10 am on August 14.

"Our officers will seek the call details of the deceased woman and those on duty on that day. They may submit the FIR in a local court," another central agency source said.

Two CBI officers on Tuesday evening went to the Tala police station here and took possession of documents related to Kolkata police's investigation.