Officers said those killed were members of the organisation, backed by Nepal Maoists, and that they were working on the bombs to be used in the blasts when a powerful timer device went off accidentally.

The police arrested the fourth member of the group when he was trying to get back to Nepal on Thursday night. The police had laid a trap at Panitanki — the gateway to Nepal about 35 km from Siliguri town. The man had sustained burns in the blast.

During interrogation, he said he had come from a refugee camp in Nepal that sheltered a large number of Nepali-speaking people who had earlier been ousted from Bhutan. “Bhutan refuses to take them back and most of them are still sheltered in the refugee camps with aids from the United Nations Human Rights Council,” an officer said. Bhutan has been hit by frequent blasts in recent times.

The three others who died in the explosion had also come from different refugee camps. The four had taken a house on rent in Siliguri, the police said.