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.

The Maine Gun Safety Coalition is asking state lawmakers to ban ghost guns after an untraceable firearm was used by the alleged UnitedHealthcare CEO shooter. Firearms known as “ghost guns” do not have a serial number as they are usually constructed at home with a kit or with the assembly of separate pieces.

Maine’s Gun Safety Coalition is calling for a ban on ghost guns following the United HealthCare case. Ghost guns are unserialized and untraceable that can be purchased as a kit or separate pieces and assembled into fully functioning firearms.

Similar bills restricted untraceable firearms have failed in the Democratic-controlled Legislature, but the political ground has shifted since last year's mass shooting in Lewiston.

Similar bills restricting untraceable firearms have failed in the Democratic-controlled Legislature, but the political ground has shifted since last year's mass shooting in Lewiston.