Tribal separatists attacked an election meeting Tripura killing 11 people, including five women and a three-year-old girl late on January 26.
Eight others were seriously wounded in the attack when armed men from the outlawed National Liberation Front of Tripura (NLFT) late on Sunday entered village Mandai, 18 km east of the Tripura capital Agartala.
Most victims were members or supporters of the state's ruling Communist Party of India-Marxist (CPI-M).
"The militants went on a rampage firing indiscriminately with automatic weapons on a group of people assembled inside a house for election campaigning," a senior police official said by telephone from Agartala.
Six people died instantly and five succumbed to their injuries early Monday in a hospital.
"Four of the injured are battling for survival with multiple bullet injuries," a doctor at the G.B Pant Hospital in Agartala said.
"As is the practice, our leaders were on a house-to-house campaign drive when the attack took place," said Gautam Das, a CPI-M spokesman.
"The attack is an attempt by NLFT to create panic and fear among our workers and supporters at the behest of opposition parties."
The NLFT, fighting for an independent tribal homeland, currently operates out of bases inside adjoining Bangladesh to carry out hit-and-run strikes on Indian soldiers.
In the past one month, at least a dozen CPI-M leaders of the state have been killed and five kidnapped by NLFT.
"Most of the killings and attacks were selective with the NLFT identifying the target beforehand," the police official said.
NLFT militants on Friday burnt down a vital road bridge in the southern district - the latest in a string of attacks ahead of the elections.
At least 200 people have died in rebel attacks during the past year, with the NLFT and the All Tripura Tiger Force (ATTF) also kidnapping and extorting money in remote areas.
Tripura Chief Minister Manik Sarkar says the terrorists were taking advantage of the 856-km unfenced border the state shares with Bangladesh.
"The porous border helps the militants in carrying out inhuman attacks and then sneaking back to their bases inside Bangladesh," he said.
A raging insurgency has killed around 10,000 people in Tripura during the past two decades.