At least 61 people were killed and more than 300 injured Thursday in a dozen blasts that ripped through towns and markets in the insurgency-hit northeastern Indian state of Assam. A police spokesman confirmed a total of 12 explosions within the space of about one hour, six of them in the state's main city of Guwahati. Three other districts in western Assam were also hit. There was no immediate claim of responsibility, although state officials said the bombings may have been the work of the rebel United Liberation Front of Asom (ULFA), which has been fighting for an independent homeland since 1979. Police were also investigating whether Islamist fundamentalist groups active in Assam -- such as the Bangladesh-based Harkat-ul-Jihad-al-Islami -- may have been to blame. "Jihadi forces could be behind this in collaboration with local militants," said state police intelligence chief Khagen Sharmna. The ULFA released a statement denying it was involved in the blasts. Assam Chief Minister Tarun Gogoi, who put the death toll at 61 with 300 injured, described the attacks as "an act of cowardice... designed and carried out to spread terror."