The letters were found near the stairs at the hostel entrance
Policemen are on a round-the-clock vigil at a missionary school in north Bengal after it received four handwritten letters in Hindi threatening to burn down the school and a nearby church if the nuns who run it did not leave immediately.
Security has been stepped up at St Capitanio Girl's Higher Secondary School, about 70 km from Jalpaiguri town, said Ravindranath, a senior police officer.
The headmistress of the school - who is a nun - filed the police complaint on Sunday about the threatening letters found near the girls' hostel.
The first of these letters was found on March 11, three days before the attack on a convent school at Ranaghat in south Bengal, where a 72-year-old nun was raped.
The letters were found near the stairs at the hostel entrance, which is well-guarded and difficult for an outsider to access. Initially, they were seen as a prank but after the Ranaghat incident, the police began to look at them seriously.
Senior officers visited the school on Tuesday to review security.
The heavy police presence has left many students and teachers nervous. "Everyone is in a panic and there is no clear information from the principal," said Dipa Bhattacharya, one of the 30-odd teachers in the school.
The school is over 50 years old and has around 1,700 students, almost 200 in the hostel. There is a missionary-run boys' school nearby as well.
In recent months, there have been many attacks on churches and Christian buildings across the country.
"I will go to any extent to protect minorities. I will do everything to end the sense of insecurity prevailing among minorities," said Home Minister Rajnath Singh on Monday.
On Tuesday, four men were arrested for allegedly vandalising a church near Mumbai over the weekend. A group of activists attacked a church and a convent in Jabalpur in Madhya Pradesh last week.