The autopsy confirmed the cause of death Monday, police said, but no one was in custody for the crime.
The victims -- Tanuja Borkar, 11, and her sisters Prachi, 9, and Priya, 6 -- had been missing from their home since Feb. 14, Gulf News reported.
Their school bags and footwear were found two days later near the well where their bodies were later discovered.
The district superintendent said the family didn't have any enemies but that the attackers could have been acquaintances.
Police said they have questioned hundreds of people about the incident.