Indian police detained three Muslims on Saturday after a little-known Islamist group claimed responsibility for bombings that killed 77 people in the troubled state of Assam. A police official said that a car and mobile phones used to detonate bombs in the remote northeastern state, including main city Guwahati, had been traced to the three men."We, ISF-IM, take responsibility of Thursday's blast. We warn all of Assam and India of situation like this in future," the text message said. Police say the group may have seeking to avenge attacks on Muslim settlers by indigenous tribes that killed at least 47 people last month. Security was tightened in Guwahati on Saturday ahead of a visit by Prime Minister Manmohan Singh to meet families of the victims and survivors. A Hindu-nationalist group linked with India's main opposition Bharatiya Janata Party called a day-long shutdown across Assam on Saturday to protest the bombings.