Here are 10 developments in this big story:
Men, women and children were shot dead with AK-47 bullets in five attacks within the space of an hour in the Sonitpur and Kokrajhar districts, the police said.
Those killed were mostly tribals who worked in tea gardens in the region. The number of dead could rise as many people were critically wounded. Home Minister Rajnath Singh will visit the state today.
Prime Minister Narendra Modi has called it an "act of cowardice". In a tweet this morning, President Pranab Mukherjee said, "Such acts of terror and violence must be put down with a firm hand."
In one village in Sonitpur, 27 people were killed. 10 women and 13 children are among the dead.
Villagers who escaped told the police that the attackers, armed with assault rifles and wearing military uniforms, forced open the doors of their thatched huts and started firing. Some people were dragged out of their homes and shot at point-blank range.
The police suspect the involvement of a breakaway faction of the National Democratic Front of Bodoland, which has been fighting for a separate homeland for decades. The Bodos are an indigenous tribe in Assam, making up 10 percent of the state's 33 million people.
The police say the attacks could be motivated by revenge. Lately, the group has suffered heavy casualties in a crackdown by security forces. Two militants were killed in the last encounter with soldiers on Sunday.
The police also say the attack could be a tactic to strike terror ahead of the elections to the Bodoland Autonomous Hill Council. The Bodo militants accuse tribals of helping the security forces.
Assam is on red alert and parts of the state are under curfew. A section of the international border with Bhutan in lower Assam has been sealed.Additional forces have been sent to the Bodoland Territorial Area District that spans Kokrajhar, Baksa, Udalguri and Chirang.
The Bodo group had last struck in May, killing dozens of Muslim settlers, which led to heavier police presence in Muslim neighbourhoods.