"We have formed an SIT to probe the Sunanda Pushkar case," Delhi Police Chief B S Bassi said today.
The police commissioner also did not rule out questioning Tharoor, saying "whatever necessary will be done" in investigating the case.
Asked why a case of murder was registered almost a year after her death, Bassi said the final medical report from AIIMS warranted the FIR to be filed so that Sunanda's viscera samples can be sent abroad for further tests.
The investigators have decided to send her viscera samples to a laboratory either in Britain or the US to identify the poison including whether it could be radioactive isotopes that cannot be detected in Indian labs.
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Sources said a Delhi Police team visited a hospital in Thiruvananthapuram in Kerala recently where Sunanda was admitted days before she was found dead in a five-star hotel in Delhi.
The SIT is likely to question Tharoor, his relatives and personal staff along with the employees of the five-star hotel where she was found dead on January 17 last year, the sources said.
It will also examine the hotel doctor who declared Sunanda dead and go through CCTV footage of the hotel. The forensic report of Sunanda's mobile phones and laptop which were sent for tests will also be evaluated.