At least one person was killed while three other sustained serious injuries after militants opened fire on a group of people in north-eastern Indian state of Assam, police said Sunday. Police said the incident happened in Mulandubi area in Kokrajhar district of Assam when armed militants belonging to National Democratic Front of Bodoland lobbed a grenade and fired upon a group of people. Police said the attack left one person dead while three others sustained injuries, adding, the injured have been hospitalised for treatment. Police said the militants escaped from the spot on a motor bike after the attack. Soon after the attack the police and additional contingents of other security forces reached the spot and start search operations to trace out the accused militants. The police has registered the case against the militants and started investigations into the incident. Several insurgent groups are active in Assam. Some of them call for regional autonomy while others demand complete independence from Indian rule.