Luigi Mangione Pleads Not Guilty to First Degree Murder in NY State

Luigi Mangione says he's not guilty of slaying Unitedhealthcare CEO Brian Thompson -- he entered not guilty pleas Monday morning in New York State court.

Mangione pleaded not guilty to 11 state charges ... including first-degree murder, 2nd-degree murder as crime of terrorism and multiple weapons charges.

The 26-year-old Ivy League grad was dressed like a college preppie -- a maroon sweater over a white collared shirt and khaki pants -- as he stood before the judge with his attorney Karen Friedman Agnifilo.

Mangione faces the possibility of life in prison without parole if convicted for murdering Thompson last month as he entered the midtown Hilton hotel.

This was Mangione's 2nd time entering not guilty pleas -- he appeared in federal court last week to face charges that could result in him getting the death penalty, if convicted.

As for which trial goes first -- officially, the feds and the Manhattan D.A.'s Office are saying the state's case will take precedence.

Mangione's alleged murder of Thompson set off a days-long manhunt after he fled the crime scene by bicycle, and made it out of NY altogether.

He was arrested at a McDonald's in Altoona, Pennsylvania when an employee called cops, after a patron recognized him in the restaurant from widely distributed suspect photos.

During their investigation, cops found a diary they say belongs to Luigi ... which shows he seemingly planned this attack for a long time -- and also pointed to Luigi's distrust of the American corporate system.

He's currently being held in MDC Brooklyn -- the notorious jail where Diddy is locked up -- while he awaits trial.

Mangione's lead attorney declined to comment after his federal court appearance, and Agnifilo was also tight-lipped Monday as she entered court for the arraignment.