Patna: In a startling twist to a Rs 1 crore extortion case in Siwan, police arrested a rural works department (RWD) assistant engineer for allegedly sending a threat letter to his senior officer, demanding the money and warning of an AK-47 attack if it was not paid.The accused, Rafiq Alam, was posted as assistant engineer, Sadar-1, in the department’s Siwan-1 works division. Siwan Town police station SHO Avinash Kumar cracked the case within 48 hours of the FIR being registered and arrested him on Thursday.RWD executive engineer Rajesh Kumar Chaudhary received the letter by post on the evening of Nov 10. The sender identified himself as Loha Singh, a Maoist area commander of the Tirhut region, and demanded Rs 1 crore.The letter directed Chaudhary to hand over the money on Nov 14 to a person wearing a red scarf, waiting in a black Scorpio on the road towards Saguna Mor in Patna. It threatened to riddle him with AK-47 bullets if he evaded payment or tried to act smart while travelling from Siwan to Patna.Chaudhary lodged a complaint at Town police station that evening. Police used digital and human intelligence to trace the sender to Alam, who worked in the same division.Alam, a native of Sarai Fakra Badh village under Kudra police station in Kaimur district, is the son of Azizul Rahman.“Alam was upset with the behaviour of his senior executive engineer. He had written several letters to his department, district administration and vigilance seeking legal action against the senior. When nothing happened, he allegedly sent the extortion letter,” the SHO said. Alam has been sent to judicial custody.