Gunmen on Friday opened fire on a busy market in a restive area of northeast India, killing 12 people and wounding several others, police said.
Authorities in Assam state where the attack took place blamed the outlawed National Democratic Front of Bodoland (NDFB), which waged a violent decades-long campaign for an independent homeland for the indigenous Bodo people.
Police said there were around six gunmen, one of whom was killed after the attack by security forces.
"The attack was carried out by the Songbijit faction of the outlawed National Democratic Front of Bodoland (NDFB)," Assam police chief Mukesh Sahay told AFP.
"There could be five to six militants involved in the attack. We are trying to zero in on the remaining militants after our forces were able to neutralise one."
He said the attackers also used a grenade, setting off a fire at a building in the marketplace which was packed with shoppers.
The state government said police and paramilitary reinforcements had been rushed to the scene in Balajan, around 220 kilometres (140 miles) west of Assam's main city of Guwahati and near the border with Bhutan.
Assam's recently elected Chief Minister Sarbananda Sonowal condemned the killings and pledged strong action against the perpetrators.
Northeast India, linked to the rest of the country by a narrow land bridge, has seen decades of unrest among ethnic and separatist groups.
The region is home to dozens of tribal groups and small guerrilla armies that resist rule from New Delhi.
Many are fighting for separate homelands for their tribes, and they often compete against each other.
Experts say the region's isolation from the rest of the country is economic and political as well as geographical.
Assam has been relatively peaceful in recent years, but tensions between the Bodo and other communities in the area persist, particularly over land rights.
Bodo guerrillas have in recent years launched ferocious attacks on both Muslim settlers and other tribal communities in the tea-growing state of Assam.
In 2014 thousands of people fled their homes in Assam after a series of coordinated attacks by armed rebels that left at least 69 people dead, 18 of them children.
Two years earlier, ethnic clashes in the same area claimed about 100 lives and displaced more than 400,000 people.
"It is very unfortunate because peace had actually returned to entire Bodoland area in Assam," said junior home minister Kiren Rijiju after the latest incident.
"We have to ascertain that who are exactly the people behind this dastardly act, it is very unfortunate so many people lost their lives."