An armed man was arrested Monday after driving to a security barricade outside the U.S. Capitol, prompting a police investigation into why he came to the heavily protected complex and temporarily shutting down part of the Capitol grounds.

U.S. Capitol Police said officers arrested the man at the North Barricade after discovering he had a firearm.

Firearms are prohibited on Capitol Grounds, and authorities immediately closed the area while investigators processed the scene.

“Our investigators are working to learn more about the suspect and why he drove to the U.S. Capitol ,” Capitol Police said in a statement.

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Authorities have not identified the suspect or announced any charges.

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Police remained at the scene Monday afternoon, where officers were seen removing items from a Ford Bronco parked diagonally in front of the barricade.

From a distance, the items appeared to include water bottles and other belongings.

A dog that had been inside the vehicle was removed safely and taken away in an SPCA vehicle.

The North Barricade remained closed as the investigation continued.

Fox News’ Dan Scully, Chad Pergram, and Kelly Phares contributed to this report.