A California man accused of attacking the White House Correspondents' Dinner and attempting to assassinate the President pleaded not guilty Monday in federal court in Washington, D.C., facing four criminal charges.

A suspect in an attack at the White House Correspondents' Dinner on Saturday night was prepared for a mass casualty event, according to prosecutors' filing in federal court. U.S. Attorney Jeanine Pirro and her office submitted a memorandum to a judge regarding the case.