BJP leader Mukul Roy was among the four people booked on Sunday in connection with the murder of Trinamool Congress (TMC) MLA Satyajit Biswas in West Bengal's Nadia district, police said.
Roy, on his part, said the decision to name him in the FIR was "politically motivated". A West Bengal Police officer told PTI that two of the four people named in the FIR have been arrested.
"So far, we have arrested two persons in this case and detained another three. A country-made revolver used to shoot the MLA has also been recovered.
"As per our primary findings, it seems that the victim was fired from behind and the act was the result of a well-planned plot," he said.
Asked about the chances of the assailants fleeing the area, the officer said the state police has been put on a high alert.
"Nadia shares border with Bangladesh and there could be a possibility of them (assailants) trying to escape to the neighbouring country. Police have been put on high alert to check the movements along the border," he said.
Satyajit Biswas (41), who represented Krishnaganj constituency in the Assembly, was shot from point-blank range inside a Saraswati puja marquee in Phulbari on Saturday.
He was immediately taken to a local hospital where doctors announced him "brought dead".
His body was brought home from the hospital Sunday morning for last rites, amid a blame-game between the ruling party and opposition BJP leaders over who was responsible for the death.
A large crowd gathered to pay last respects to the deceased MLA at his residence, a few metres away from the Saraswati Puja marquee where he was shot.
Roy, a former TMC member of Parliament, said the legislator's killing could be a result of infighting within the TMC.
Roy joined the BJP last year after his relations with Chief Minister Mamata Banerjee soured.
"Naming me in the FIR is absolutely a politically motivated move. I am sorry about Satyajit's death, but the manner in which they (TMC leaders) are trying to hold the BJP responsible is nothing but a dirty conspiracy," Roy told PTI.
Roy said Biswas was surrounded by TMC party workers and activists when he was shot dead. "You have to think about it. Satyajit was surrounded by his partymen when he was shot. How did the killers manage to go near him? It is quite clear as to what happened."
TMC secretary-general Partha Chatterjee, however, alleged that those who killed the MLA belonged to the saffron party.
Along with party's Nadia district observer Anubrata Mondal, who asserted that the "none of the perpetrators will be spared", Partha Chatterjee accompanied the MLA's hearse to his residence from the hospital.
Partha said the chief minister was shocked to learn about the incident. "Those who think that they will gain by eliminating Satyajit, are living in a fool's paradise," he said.
Urban Development Minister Firhad Hakim also squarely blamed the BJP for the killing of Biswas, who belonged to the Matua community - deemed vital as a support base in Bengal.
"The prime minister came to woo the Matua community in Thakurnagar last week. Now the local BJP leaders are resorting to criminal activities to gain political turf," he said.
Denying allegations of BJP's involvement, the party's state president, Dilip Ghosh, demanded a CBI investigation into the killing.
An independent probe will reveal the truth, he said.
Ghosh echoed Roy's sentiments saying Biswas was a victim of an internal feud within the ruling party.
"If an MLA gets killed like this, how can the common feel safe?" he asked.