The police's FIR or First Information Report does not name anyone.
Describing Ms Pushkar's death as "unnatural", Delhi Police Commissioner BS Bassi said, "We registered the murder case on the basis of a medical report which says that poison consumed by the victim was the cause of death. Poison was either given orally or injected in the body."
Viscera samples, he said, are being sent abroad for investigation.
Police sources told NDTV that new facts have emerged in a medical report received on December 29, from Delhi's All India Institute of Medical Sciences (AIIMS), that necessitated a murder inquiry. The report rules out natural death and also throws new light on injuries on Sunanda Puskhar's body, the sources said. (New Facts Necessitated Murder Case: Delhi Police on Sunanda Pushkar)
"We will interrogate and investigate whosoever required," Mr Bassi said, when asked if Ms Pushkar's husband Shashi Tharoor would be questioned in the case.
In a statement, Mr Tharoor said, "I am stunned to hear that the Delhi Police have filed a case of murder against unknown persons in the demise of my late wife Sunanda. Although we never thought of any foul play in the death of my wife, we all want that a comprehensive investigation be conducted and that the unvarnished truth should come out." (Read the full statement here)
Sunanda Pushkar was found dead in a five-star hotel in Delhi on January 17 last year, a day after she accused a Pakistani journalist of having an affair with Mr Tharoor, in an angry Twitter exchange.
Mr Tharoor's party, the Congress, said today that, "Everybody who is remotely connected should be questioned." The party's spokesperson, Abhishek Manu Singhvi, also said, "Let us maintain a sense of balance. This is a complaint/FIR against persons unknown, not yet clear whether its suicide or murder. We must not sensationalize this. Let us wait." (Not Clear if Sunanda Pushkar Was Murdered: Congress)
Congress leaders like Mr Singhvi and Rashid Alvi have also asked why took the police a year to register a case.
"Proper investigation was required which we did," said Mr Bassi.
Sunanda Pushkar was an entrepreneur based in Dubai before she married Shashi Tharoor in 2010.
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