A man with a loaded handgun and more than 500 rounds of ammunition has been arrested in Washington at a security checkpoint near the US Capitol, authorities said.

Wesley Allen Beeler, of Virginia, had driven to a checkpoint on Friday evening and tried to use a phony credential to access the restricted area where President-elect Joe Biden will be inaugurated next week, according to a document filed in Washington, DC Superior Court.

As officers checked against an authorized access list, one of them noticed decals on the back of Beeler’s pick-up truck that said “Assault Life,” with an image of a rifle, and another with the message: “If they come for your guns, give ’em your bullets first.”

Under questioning, Beeler told officers he had a Glock handgun in the vehicle. A search uncovered a loaded handgun, more than 500 rounds of ammunition, shotgun shells and a magazine for the gun, the court document said.

Beeler was arrested on charges including possession of an unregistered firearm and unlawful possession of ammunition, a police report said.

Following his arrest, Beeler said it was “an honest mistake” and that he was a private security guard who got lost on his way to work near the Capitol.

Washington is under a high state of alert ahead of Biden’s Wednesday inauguration after a mob of President Donald Trump’s supporters stormed the Capitol on January 6.

Five people died in the assault, including a police officer.

Security officials have warned that armed pro-Trump extremists, possibly carrying explosives, pose a threat to Washington as well as state capitals over the coming week.

Thousands of National Guard troops have been deployed in Washington and streets have been blocked off downtown with concrete barriers.

The National Mall, which is normally packed with people every four years for presidential inaugurations, has been declared off-limits at the request of the Secret Service, which ensures the security of the president.