Mr Laskar represents the Sonai constituency in Cachar district of Assam. He said he received the letter by post. He also received two live bullets. "It (the letter) said the BJP and the RSS are communal organisations and they are working against the Muslims. So, being a Muslim, I should not be in that party," he said.
The Silchar police said the letter came from an unknown group 'Save Secure and Development Protection Force of Muslim, Barak Valley Zone'.
"We have registered an FIR against unknown persons for a threatening letter to Laskar on Sunday. Two live cartridges of .32 pistol also came with the letter," Silchar Police Station Officer-in-Charge Indrajit Chakraborty said.
The case was registered under Sections 153A (promoting enmity between different groups on grounds of religion, race, etc) and 506 (criminal intimidation) of the Indian Penal Code.
Mr Laskar, however, doubted the authenticity of the letter, saying it might be the handiwork of urea, drugs and land mafias "against which I have been fighting".
The lawmaker said he will continue his fight against anti-social elements and demanded a thorough probe by the police.