The nun, who was admitted to a hospital post the incident and was critical, has undergone a surgery and her condition has stabilised say doctors.
The dacoits entered the convent and gagged the lone security guard on duty after which they attacked the nun and allegedly gang-raped her, said Nadia district police and Criminal Investigation Department at a joint press conference today.
Rs 8-9 lakh of administrative funds including students fees was also looted and church ransacked, said police.
Four of the six dacoits have been identified through CCTV footage and the police have announced a reward sum of Rs 1 lakh for any information that leads to their arrest.
Chief Minister Mamata Banerjee has ordered a probe by CID into the incident. "We strongly condemn the incident that took place at the Ranaghat Convent... CID will be examining all aspects to this horrific crime. Swift, strongest action to be taken," said Ms Banerjee in a series of tweets posted by her party, the Trinamool Congress.
CID will be examining all aspects to this horrific crime. Swift, strongest action to be taken: WB CM- AITC (@AITCofficial) March 14, 2015
"After tying, 3 of the 5 Sisters, they entered the Principal's room and ransacked it, destroying property and taking money, a laptop and a mobile phone," Fr. Dominic Gomes, Vicar General of Kolkata Archdiocese, said in a statement.
Angry school students today blocked trains on the Sealdah-Ranaghat route and National Highway 34 in protest against the incident.
The State Urban Development Minister Firhad Hakim termed it as an "attack on humanity".
"A religious fanaticism has been created in the country. Different churches are being attacked at various places. In the name of 'Ghar Wapsi', people are being religiously provoked," he said.