Luigi Mangione pleaded not guilty Friday to a federal murder charge in the killing of UnitedHealthcare CEO Brian Thompson as prosecutors formally declared their intent to seek the death penalty against him. Mangione, 26, stood with his lawyers as he entered the plea, leaning forward toward a microphone as US District Judge Margaret Garnett asked him if understood the indictment and the charges against him. Mangione said, “yes.” Asked how he wished to plead, Mangione said simply, “not guilty” and sat down. Mangione’s arraignment for the killing last December attracted several dozen people to the federal courthouse in Manhattan, including former Army intelligence analyst Chelsea Manning, who served about seven years in prison for stealing classified diplomatic cables. Story continues below this ad Mangione, who has been held in a federal jail in Brooklyn since his arrest, arrived to court in a mustard-colored jail suit. He chatted with one of his lawyers, death penalty counsel Avi Moskowitz, as they wanted for the arraignment to begin. Late Thursday night, federal prosecutors filed a required notice of …