The gun used in last week's fatal shooting of UnitedHealthCare CEO Brian Thompson may have been created with a 3D printer.

Towson native Luigi Mangione, who was charged in the shooting death of a prominent health insurance CEO, allegedly was in possession of a “ghost gun” when he was arrested Monday, putting the spotlight on a way of obtaining a firearm that has policymakers and gun violence prevention advocates increasingly concerned.

Ghost guns' have been a growing concern for years now, and Mangione's arrest Monday put them back in the spotlight.

Police say the suspect had a ghost gun, possibly made with a 3D printer, when he was arrested on Monday. Here's what to know about the increasingly widespread weapons and efforts to regulate them.