Kolkata/New Delhi: In the days before the rape of an elderly nun in an armed assault that has shaken India, death threats and extortion attempts had already shattered the peace at her Convent of Jesus and Mary school, police and school officials say.

For 19 years, the Catholic school in the town of Ranaghat in West Bengal had been teaching children of workers at a nearby jute factory. But three weeks ago, the school received death threats and demands for money in anonymous phone calls just days after the nuns had an altercation with the father of a boy who was expelled.

Police are investigating if there was a link between the expulsion of the boy, the threats and Friday night's attack, when a group of ten men broke into the school offices before heading to the nuns' quarters.

"This is a well-planned attack and conspiracy cannot be ruled out," said a police officer in Ranaghat contacted by telephone, who asked not to be named because of the sensitivity of the case. He said it was not yet clear if the three incidents were linked and that police were also investigating the religious angle.

The assault marks a disturbing convergence of two social currents in contemporary Indian, violence against women and attacks against minority religions.

A rape is reported on average every 21 minutes in India, and acid attacks, domestic violence and molestation against women are common.

Since December, half a dozen churches have been vandalised, at the same time as conservative groups have campaigned to convert members of "foreign religions" such as Islam and Christianity to Hinduism.

In the Ranaghat attack, the men roamed around the school and damaged a bust of Jesus Christ in a chapel before breaking into the nuns' quarters. Two attackers grabbed the 75-year-old nun who was in charge of the school's finances and demanded the keys to a cupboard, the officer said, citing a report given by another nun who was an eyewitness.

When she refused, she was raped by one man, the officer said. Other nuns were threatened with similar consequences unless they handed over gold chains and cash. In total the men made off with more than 1 million rupees.

THREATS AFTER EXPULSION

Police have detained nine men, but no arrests have been made.

"We have never had to face a painful situation like this in India," said Sister Janet, who supervises a network of 40 Convent of Jesus and Mary schools in India.

She said nuns in the school told police about the telephone threats in late February. They came days after the nuns argued with the father of a boy who was expelled for posting photos of a girl student online and making lewd remarks on her Facebook page.

Rather than opening an investigation into the threats, the police registered their concerns in a book reserved for minor complaints, Sister Janet said.

"Police should have provided protection to the nuns but they did not take the complaint seriously," she said.

PM Modi said on Tuesday he was deeply concerned about the rape.