The girls, ages 14 and 16, were cousins and were attacked at a village in Uttar Pradesh state's Budaun district, said police deputy inspector-general R.K.S. Rathore.
"Both the attackers and the victims were of backward castes and from the same village ... We are investigating all possible angles and collecting evidence. So far, we have no evidence that points to any kind of enmity," he said.
Police have arrested one man and charged him in the deadly assault, and are searching for two additional suspects.
A police constable was also arrested for allegedly obstructing the investigation by siding with the suspected attackers, and delaying the case registration.