Kolkata, March 16 (IANS): The septuagenarian nun who was raped by a gang of bandits in West Bengal's Nadia district, is physically stable but mentally vulnerable, doctors attending her said on Monday.

The nun needs some more time to overcome the trauma, they added.

The sister superior of the Convent of Jesus and Mary High School in Ranaghat - some 80 km from here - was gangraped by the bandits in the early hours of Saturday, sending shock waves across the country and even beyond.

Director General of Police G.M.P. Rajasekhar Reddy on Monday visited the convent and held talks with his officers even as police are yet to arrest any of the culprits.

Admitted at the Ranaghat Sub Divisional Hospital, the nun is responding to treatment.

"A team of doctors comprising a physician, a gynaecologist, a surgeon and a psychiatrist examined her this morning (Monday). Physically, she continues to be critical but her condition is improving and she is responding to treatment," assistant hospital superintendent Payal Mullick Lala said.

"However, mentally we cannot say anything. She still needs some more time to overcome the trauma," said Lala.

Chief Minister Mamata Banerjee has handed over the investigation to the Criminal Investigation Department.

Police on Sunday released snapshots of four of the culprits from CCTV footage and announced reward of Rs.1 lakh for information on them.

"So far we have detained and interrogated 8-10 people but we are yet to make any arrests. We are checking their criminal antecedents and matching their pictures with those of the four criminals caught in the CCTV footage," Superintendent of Police (Nadia) Arnab Ghosh said.

Ghosh said the four whose snapshots have been released "prima facie" appeared to be the main culprits.