Police investigating the rape of an elderly nun in the Indian state of West Bengal say they have arrested a key suspect in the crime.

The police allege 28-year-old Nazrul, also called Nazu, was the man who raped the 74-year-old nun.

The man, allegedly a Bangladeshi citizen, was arrested at a railway station in Kolkata (Calcutta).

The incident caused outrage in India and led to street protests in many cities across the country.

This marks the sixth arrest in the crime.

Police have identified eight suspects in the attack, of which two still remain untraced.

"All the five accused arrested earlier have named Nazrul as the person who raped the nun," a police officer told the BBC.

Dilip Kumar Adak, deputy inspector general of the state's Criminal Investigation Department, told the AFP news agency that police acted on a tip off that Nazu, who had fled to Bangladesh after being identified from CCTV footage of the attack, was due to return to Kolkata by train.

In May, the police said they had arrested Milon Sarkar and described him as the leader of the gang which attacked the convent. They said it was not clear what role the man had played in the case.

During the attack in Ranaghat town on 14 March, money was stolen from the convent school and the building ransacked, before the nun was raped in the convent itself.