Mr Modi, in a video message on Twitter, showed the content of the letter.
Former Bihar deputy chief minister and senior BJP leader Sushil Modi on Tuesday said he received a death threat from a person who claimed to be a TMC leader from Bengal.
Mr Modi also said that he forwarded the threat letter to Senior Superintendent of Police (SSP), Patna, Manavjit Singh Dhillon.
The sender, who signed as 'Champa Some (Soma)', claimed to be resident of Purba Bardhaman district of Bengal, the BJP leader said.
The letter said, "I am informing you that I am a leader of Trinamool Congress. Mamata Banerjee may become the next Prime Minister of India... Mamata Banerjee and Nitish Kumar Zindabad. I will kill you."
Mr Modi, in a video message on Twitter, showed the content of the letter.
He added that it was sent by speed post.
Talking to PTI, Mr Dhillon later said, "We have received a complaint from the former deputy CM and have initiated the process of registering an FIR to probe the matter."
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