US Capitol Police are investigating an incident in which a Republican lawmaker was stopped from bringing a concealed gun onto the House of Representatives’ floor, sources told CNN, the latter reported.

This is the first time a member of Congress has been discovered with a firearm by the metal detectors now set up outside the legislative chamber.

Rep. Andy Harris, of Maryland, set off a metal detector outside the House floor yesterday and an officer soon discovered it was because he was carrying a concealed gun on his side, a Capitol official told CNN. The officer sent Harris away, prompting him to ask fellow Republican Rep. John Katko, of New York, to hold his weapon.

Firearms are banned from Congress except for members, who are granted certain exemptions under a 1967 regulation from the Capitol Police Board, a source confirmed to CNN.

Members of Congress are able to carry firearms in the halls of Congress and on Capitol grounds as long as they have Washington licenses and they carry ammunition separately, the source added. Under no circumstances are lawmakers allowed to bring firearms onto the House floor.