Police detained three people 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.
Thursday's coordinated bomb attacks were the worst in India's turbulent northeast, home to more than 200 tribes and a focus of dozens of insurgencies connected with demands for autonomy or statehood.
A little-known Islamic group, Islamic Security Force-Indian Mujahideen sent a mobile text message to a local TV station claiming responsibility for the bombings.
"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.
The three men were detained in Nagaon district, about 125 km (75 miles) east of Guwahati.
"Police is probing if they have any involvement in the serial blasts," said J. Balaji, Nagaon district's chief administrator.
Ethnic tensions have simmered for decades in Assam where over the years Muslim settlers, mostly from Bangladesh, have moved to this Hindu and tribal-dominated region.
Security officials said they were investigating if the ISF-IM group was the same as an Islamic militant organisation formed in 2000 in Assam to avenge attacks by indigenous people.
Police were also investigating the links of ISF-IM with the Indian Mujahideen, which first emerged in Nov. 2007 and has claimed several major attacks across India this year.
The separatist United Liberation Front of Asom (ULFA) is also suspected, but security experts say the scale and sophistication of Thursday's attacks bore the hallmark of Islamist militant groups, and that ULFA may have only played a supporting or logistical role. ULFA has denied any involvement.
Police were also trying to find out if ISF-IM was a front for Bangladesh-based Harkat-ul-Jihad al Islami (HuJI) that has often been suspected of attacks on Indian cities. HuJI has ULFA links.
(Writing by Krittivas Mukherjee; Editing by Sanjeev Miglani)