The outlawed United Liberation Front of Asom (ULFA), the dominant militant group in the region, was blamed for the explosion, which also left 24 people injured.
The ULFA has been fighting for an independent homeland in Assam since 1979. Police say they have completed their investigations and are waiting for government approval in order to prosecute top rebel leaders for the killings.
Security too has been stepped up with about 50 000 army and paramilitary troops deployed across the region.
But as militant groups announce their annual boycott of the celebrations to protest New Delhi's rule about the vast region rich in oil, tea and timber, parents are keeping their children indoors and out of harm's way.
Seven rebel groups issued a statement this week vowing to continue their "struggle for liberation from Indian colonial occupation."
"We call upon the entire people of our region to boycott and prevent the celebration, in any manner, of India's Independence Day in our region," said an email sent by the separatists to local dailies.
Late on Wednesday, unidentified militants hurled four powerful grenades at the private residence of a minister in neighbouring Manipur state.
Vital installations, including oil pipelines, trains, roads and bridges face renewed attack at this time too.
Against the backdrop of the expected surge in violence, the sounds of a police brass band rehearsing in Dhemaji ahead of India's 61st celebration of freedom from British rule brought little cheer.
"The image of Girin comes alive when we hear the sound of the brass band and that literally makes us mentally imbalanced," said Ajit Saikia, a middle-aged businessman whose 14-year-old son Girin died in the 2004 landmine attack.
Government engineer Puspa Deuri, who lost his wife that day, feels the same way.
"Whenever this day approaches I feel as if my heart is going to stop beating," he said.
At least 50 000 people have been killed in separatist violence across the north-east since India won its independence in 1947