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 state had been teaching children of workers at a nearby jute factory without incident. 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.

The attack also comes amid a trend of worsening security for India's Christian minority, less than one year since the government of Hindu nationalist Prime Minister Narendra Modi came to power.

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.

While the rape and robbery did not have clear religious overtones, it took place against the backdrop of Hindu nationalistic rhetoric from groups allied with the government that critics believe provides a sense of impunity for attacks against minorities.

The assault also 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.

About a fifth of India's 1.27 billion people identify themselves as belonging to faiths other than Hinduism. Muslims make up the largest minority, about 14 percent of the population, while Christians comprise about 2.5 percent.

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 ($15,950).

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.

Modi said on Tuesday he was deeply concerned about the rape. Hardline Hindu nationalist groups that support him also condemned the rape, but said it would not affect their mission.

"We speak the truth and mobilise support to fight for the well-being of a Hindu society," said Surendra Jain, spokesman for the radical Vishwa Hindu Parishad or World Hindu Council.

"Why should we stop from promoting what we believe is the absolute truth? This is a Hindu nation and Christians and Muslims should acknowledge this fact."

MOTHER TERESA

The leader of the Hindu nationalist Rashtriya Swayamsevak Sangh (RSS), a group that is the ideological parent of Modi's ruling party, said in February the charitable work of Mother Teresa was aimed at conversion.

While there is no suggestion that the school attack was carried out because of this, some commentators have noted that Mother Teresa, a Nobel peace laureate who has been beatified, also was a Catholic nun doing charitable work in West Bengal.

Despite a commitment to the preservation of the Catholic faith, and religious classes for Catholic children, many of the Ranaghat school's 800 pupils are Hindus.

"If they would be converting our children then none of the Hindus would have ever admitted their children to this school," said Abiram Gupte, whose three sons study there.

"We cannot blame anyone," said Sister Janet. "But the statements by leaders of hardline Hindu organisations do make an impact on the society, they want people to start viewing our charitable work as a strategy to expand Christianity."