Police investigators say they 'should be able to zero in on' those behind the series of blasts [AFP]

A total of 13 explosions ripped through the northeastern state on Thursday, six of which were detonated in the state's capital city, Guwahati.

A senior official, who did not want to be identified, claimed on Friday that investigators were making progress with the investigation.

"We should be able to zero in on the people or groups involved in the serial bombings,' he said.

No one has claimed responsibility for the string of attacks, although state officials said the separatist United Liberation Front of Asom (ULFA) may be behind the bombings.

Police were also said to be investigating whether Islamic groups active in the region, including the Bangladesh-based Harkat-ul-Jihad-al-Islami, could have masterminded the attacks.

'Needle of suspicion'

Speaking in the immediate aftermath of the blasts, Himanta Biswa Sarma, a government spokesman, said "the needle of suspicion" pointed to the ULFA.

ULFA, however, denies it had any involvement in the bombings.

Preliminary investigations showed some of the bombs had been strapped to motorcycles and bicycles and packed with incendiary materials designed to trigger fires and maximise damage.

Bhaskar Jyoti Mahanta, Assam's inspector general of police, cautioned that the investigation was "at a very preliminary stage" but said: "We have reason to believe that the militants used PE-3 explosives, a highly plasticised, RDX-based explosive."

"Most of the bombs were planted in crowded places like markets and office complexes... so it shows that the perpetrators wanted high casualties," Sarma said.

Deaths overnight

Ten people died from injuries overnight, taking the death toll to 76. Figures for those wounded by the attacks range from between two and three hundred.

Guwahati took the brunt of the attacks with one bomb exploding just 100 metres from the official residence of Tarun Gogoi, the state chief minister.

More bombs exploded in Barpeta, Kokrajhar, Bongaigoan and Tinsukia districts.

Manmohan Singh, the Indian prime minister who is expected to visit the region on Friday, vowed every effort would be made to bring the bombers to justice.

He said: "Such barbaric acts targeting innocent men, women and children only highlight the desperation and cowardice of those responsible."

Many armed separatist groups are active in India's northeast, which is wedged between Bangladesh, Bhutan, China and Myanmar, with only a thin corridor connecting it to the rest of India.

Earlier this month, at least two people were killed and 100 injured in four bomb blasts in Assam that the police blamed on Muslim groups based in Bangladesh.

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