Guwahati: In Assam's worst terror attack since 2014, gunmen in military fatigues opened fire at a crowded market in Kokrajhar town on Friday, killing 13 civilians. One of the attackers was also killed. The terrorists hurled a grenade and then opened indiscriminate fire after reaching the weekly market in Kokrajhar town, some 220 km from here, in an autorickshaw, police and military officials said. Nearly 20 civilians were also injured. Both the dead and injured were mainly Bodos, who dominate Kokrajhar and the surrounding areas. An army patrol in the vicinity hit back, killing one of the terrorists. The security forces immediately launched a major operation to hunt for the other militants. Authorities blamed the "anti-talk faction" of the National Democratic Front of Bodoland (NDFB) led by I.K. Songbijit, who carries a reward of Rs 10 lakh on his head, for the massacre. "Saddened by the attack in Kokrajhar," Prime Minister Narendra Modi said. "We strongly condemn it." He added the Home Ministry was in touch with the Assam government and monitoring the situation closely. "Terrorists moving in an autorickshaw opened indiscriminate fire at Balajan Tinali area, killing 13 civilians and injuring 20 others," Defence Ministry spokesman Lt. Col. Suneet Newton said. "The army has launched a massive combing operation, pressing into service specialized troops and tracker dogs. The army will continue its relentless operations against the terrorists," he added. Assam Police chief Mukesh Sahay said the militants had chosen a soft target. "We suspect the number of attackers was four or five." Security forces recovered an AK-56 rifle and grenades from the spot. This was the worst terror attack in Assam since December 2014 when the NDFB killed more than 70 Adivasis in Kokrajhar and Tezpur districts. This led to a major crackdown against the NDFB's "anti-talk faction". Chief Minister Sarbananda Sonowal ordered a high alert in view of the Independence Day on August 15. "We will not tolerate any threat from any group," he said. "The government will not bow to any pressure while tackling terrorist groups." Sonowal, who took charge of Assam in May at the head of its first BJP government, vowed to deal with the terrorists "very sternly". He said: "No one involved in the crime will be spared." He announced ex gratia of Rs 5 lakh to the families of those killed and told the administration to ensure that the injured got free treatment and to shift the seriously wounded to Guwahati. A medical team from Guwahati was rushed to Kokrajhar. The Chief Minister told the media here that he had spoken to Modi and Home Minister Rajnath Singh and suggested that "a foreign hand" may be involved. In a bid to avoid retaliatory killings, Sonowal appealed for peace and harmony.