Dhananjay Kumar Mehta was arrested from Narayanpur village of Latehar district, 145 km from state capital Ranchi, on Friday, Sep 26.
The police said that they retrieved a pistol, some live cartridges and other documents from the man.
"Mehta issued threat letters to PM, Sonia Gandhi and Chidamabaram in Jul this year. He was also issuing press releases in name of Abhayji, who is zonal commander of CPI-Maoist (Communist Party of India-Maoist). He got a letter pad of CPI-Maoist printed," said a senior police officer.
"Mehta had called a five-day strike in Palamau region of the state by issuing a press release," said the officer.
The letter issued to Manmohan Singh and Sonia Gandhi read, "Both PM and Sonia Gandhi will meet their fate like former prime minister Rajiv Gandhi."
The police officer said that interrogation was on as to why Mehta issued such letters and press releases.