A man arrested on firearms charges in Pennsylvania was later charged in New York with murder in the killing of UnitedHealthcare CEO Brian Thompson.

Luigi Mangione, who was detained at a McDonald's in Altoona, Pa., has been identified as the suspect believed to have shot and killed UnitedHealthcare CEO Brian Thompson last Wednesday in New York City.

Police uncover suspected backpack of gunman who killed UnitedHealth CEO Brian Thompson - Security footage captured the gunman wearing a backpack during the shooting but then without it while exiting C

Luigi Mangione, who has been charged in last week's murder of UnitedHealthcare CEO Brian Thompson in New York City, was taken into custody on unrelated charges in Pennsylvania earlier on Monday.

Luigi Mangione yelled to the press about "an insult to the intelligence of the American people" as he was physically dragged into a Pennsylvania courthouse.

As investigators build their case against suspect Luigi Mangione for the killing of UnitedHealthcare CEO Brian Thompson, police say the gun found in his backpack appeared to be a ghost gun with parts created on a 3D printer.

Fingerprints taken from Luigi Mangione, the suspect in the killing of UnitedHealthcare CEO Brian Thompson, appear to match fingerprints recovered from items found near the shooting scene, law enforcement sources told ABC News.

The defense attorney representing Luigi Mangione, the suspect in the killing of UnitedHealthcare CEO Brian Thompson, said he has seen "no evidence" linking his client to the killing, adding that "a lot of guns look the same.