The 71-year-old nun of a convent school was gang-raped in Ranaghat town, some 80km from Kolkata, early on Saturday. Miscreants also looted Rs 12 lakh which was kept in the almirah locker.
Sources say that most of those detained have criminal background.
West Bengal Chief Minister Mamata Banerjee had ordered a CID probe into the incident. The government also announced that a reward of Rs 1 lakh would be given to any individual providing credible information leading to the arrest of the accused.
Police said that a study of the CCTV footage at the convent had shown them visuals of four persons who were allegedly involved in the crime.