Here are 10 points of the story:
A forensic doctor has told the police that the blood sample they were given right after the accident was not enough for a clear test.
At least 10 ml blood is needed to detect alcohol or drugs but the Ruby Hospital, where the actor was taken for treatment, apparently submitted half that amount to the forensic lab.
A senior forensic officer, Shubha Hazra, has written to the police that the sample was "in such small quantity that poison could not be detected by the available method of chemical analysis."
The failed blood sample test will not dilute the culpable homicide charge against Mr Chatterjee, the police said.
There is "enough oral evidence" from witnesses of Mr Chatterjee drinking alcohol on the night of the accident.
Mr Chatterjee, a popular TV actor, was apparently driving Ms Chauhan home after a party around 3.30 am on April 29 when the car hit a pavement and flipped.
The two were photographed in at least two clubs that night and in one photo shared by Ms Chauhan's friends, Mr Chatterjee was holding what they alleged was a drink.
The actor has strongly denied that he was drunk when the accident took place. He reportedly admitted later to the police that he had consumed some alcohol.
Though it has not been established, there are reports that the car was being driven at a high speed.
The case took on a political twist with opposition parties in Bengal accusing Chief Minister Mamata Banerjee and her party of trying to protect Mr Chatterjee, who plays the lead in a popular television show.
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