India, April 25 -- Luigi Mangione, who is accused of fatally shooting UnitedHealthcare CEO Brian Thompson, pleaded not guilty to federal stalking and murder charges on Friday. The 26-year-old appeared in Manhattan federal court, wearing prison garb over a thermal white shirt. If convicted, he could be sentenced to death.

Ahead of Mangione's plea, an expert told RadarOnline that the 'CEO-killer' isn't looking at a quick death if he is convicted. He could be drugged and put to sleep, and his death could 'take 18 minutes'.

"It's very likely that the drugs cause extreme pain and needless suffering in other words, torturing people to death," the expert told the publication.

Attorney General Pam Bondi has previously said that Brian Thompson'...