Luigi Mangione on Tuesday was formally accused of first-degree murder, a charge that branded him a terrorist and aimed directly at the idea that the killing of UnitedHealthcare’s chief executive was a legitimate political act.

“This was a frightening, well-planned, targeted murder that was intended to cause shock and attention and intimidation,” said Alvin L. Bragg, the Manhattan district attorney, at a news conference on Tuesday.

The assassination of the chief executive, Brian Thompson, on Dec. 4 in the heart of Manhattan set off a dayslong manhunt and captivated Americans, many of whom vented their frustrations on dealings with health insurance companies. Some voiced their support for Mr. Mangione and rooted for him to elude capture.

But on Tuesday, prosecutors said that Mr. Mangione’s actions were “in furtherance of an act of terrorism.” They were, prosecutors said, “intended to intimidate or coerce a civilian population” and to “affect the conduct of a unit of government by murder.”

Mr. Mangione, 26, also faces two counts of second-degree murder and weapons charges in New York in the killing of Mr. Thompson.

Mr. Bragg said that the charges were in response to the “brazen, targeted and premeditated shooting,” adding that he couldn’t think of another office “more equipped to handle a terrorism charge.”