Luigi Mangione is believed to be a person of interest in last week's deadly shooting of UnitedHealthcare CEO Brian Thompson, police said Monday. The 26-year-old was arrested on firearms charges in Pennsylvania, where New York police detectives were heading to interview him amid a massive manhunt for the shooter.

The GiveSendGo defense fund for the 26-year-old Mangione was established by an anonymous group calling itself "The December 4th Legal Committee," apparently in reference to the day Mangione allegedly ambushed and gunned down Thompson in Midtown Manhattan as the executive walked to his company's shareholders conference at the New York Hilton hotel.

Mangione’s face was known to cops in California because he’d been reported missing by his family just weeks earlier on Nov. 18, the sources added.