Amaravati: Security cover of Chief Minister N Chandrababu Naidu is being stepped up in the wake of a letter purportedly written by the outlawed CPI(Maoist) threatening to “take revenge” on him over the Malkangiri encounter this week in which 34 ultras were killed.Currently, Naidu is under ‘Z plus’ category protection and his security detail includes members from the elite National Security Guard apart from AP Police’s Intelligence Security Wing. He had survived a claymore mine attack by Naxals way back in October 2003 at Alipiri, on the foothills of Tirumala. Strangely, the letter, issued in the name of Maoist leader Shyam, spoke about “suicide attacks” which was hitherto unheard of in any of their communications. Hence, the veracity of the letter was being questioned. The letter, threatening to avenge the “Malkangiri killings” by specifically targeting the Chief Minister and his son Lokesh, was sent to a Telugu media house in Hyderabad.