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Sources said the high-profile case has been handed over to Crime Branch considering "various aspects" involving it.
On Tuesday, a sub-divisional magistrate, who probed Sunanda's death, had directed the police to investigate murder or suicide angles in the case after the autopsy report mentioned "poisoning" as the reason for her demise.
52-year-old Sunanda was found dead in a 5-star hotel in South Delhi on Friday night, a day after her twitter spat with Pakistani journalist Mehr Tarar over an alleged affair with Tharoor.
In a report to the police, SDM, who has recorded the statements of Sunanda's brother, son, Tharoor and his staff, said that no family member suspected any foul play in the death.
The sources had said autopsy report also mentions a "deep teeth bite" on the edge of the left palm apart from mentioning over a dozen injury marks.
They had said autopsy report has found mix of two drugs - Alprazolam and Excedrin - in Sunanda's body. Excedrin is a combination of acetaminophen, aspirin, and caffeine.
The SDM, in his report, had said Sunanda could have died in "just three ways, homicidal, suicidal and accidental" and police should investigate further to ascertain the reason of her death.
The SDM said under section 176 of CrPC his job was to determine cause of the death which has been established as poisoning.
Any death within seven years of marriage is legally required to be investigated by the sub-divisional magistrate.
