By Subir Bhaumik
BBC News, Calcutta
Assam has been plagued by violence since the 1970s
Tribal guerrillas have hacked four members of a Hindi-speaking migrant family to death in India's north-eastern state of Assam, police say.
The guerrillas broke into the house with guns and knives after encircling Rangapahar village, 160km (99 miles) north of the state's capital Guwahati.
Tuesday's attack is suspected of being carried out by one of Assam's many ethnic militant groups.
However police say that none have so far claimed responsibility.
"They hacked the man and his wife and two children to death," Assam police chief GM Srivastava said.
Mr Srivastava said it was not yet clear which of Assam's many tribal underground groups was responsible, but intelligence reports suggest that the separatist National Democratic Front of Bodoland (NDFB) was behind the attack.
The NDFB has been negotiating with the Indian government for the last five years, but a hardline faction led by its former chairman Ranjan Daimary continues a separatist campaign - although he has been expelled from the organisation.
Mr Daimary's supporters have been blamed for explosions in four towns in Assam on 30 October, in which 87 people were killed.
He claims to lead the "real NDFB" and says that those negotiating with the government are "traitors".
But the NDFB is not the only insurgent group to target Hindi-speaking migrants in Assam.
The United Liberation Front of Assam (Ulfa) - the state's strongest separatist group - and the Black Widow faction that fights for a separate homeland for Dimasa tribespeople has also been blamed for targeting migrants.
The Ulfa, which has alleged that Hindi-speakers worked for Indian security forces, has asked many to leave the region or face "dire consequences".
Last year 96 migrants were killed, mostly small traders or labourers working in farms or brick kilns.
About 30 groups in the north-east have been fighting for decades for independence from India or for greater autonomy in a state that is sandwiched between China, Burma, Bhutan and Bangladesh and is only joined to the rest of India by a narrow 22km (13.6 miles) corridor.