"Considering the seriousness and sensitivity of the case and also the fact that the place of incidence is very close to border area, I have decided to entrust the investigation of the case to the CBI," Chief Minister Mamata Banerjee said in a Facebook post.
She said her government would provide all necessary cooperation and assistance to the CBI for investigation in this case.
The state government had earlier ordered a CID probe into the incident.
No arrest could be made so far although 15 persons have been detained in this connection, the police said.
President of Catholic Bishops Conference of India (CBCI) Baselios Cardinal Cleemis, who arrived at Ranaghat to express solidarity with Sister Superior of a convent school who was allegedly gangraped, said, "CBCI would like to request the authorities to ensure that justice is done".