U.S. Capitol Police arrested a congressional staffer Thursday after he walked into a legislative building with a gun in his bag, leading to a brief lockdown in a Capitol Hill complex still on edge nearly a year after a deadly insurrection.

Jeffrey Allsbrooks, 57, was taken into custody about four minutes after he entered the Longworth House Office Building, police said. Officers who were stationed at a security screening checkpoint at one of the building’s doors “spotted an image of a handgun in a bag on the X–ray screen,” Capitol Police said.

Officers tracked down Allsbrooks, who works in the office of the House Chief Administrative Officer. Police said Allsbrooks told them that he forgot the gun was in his bag,

It’s unclear why Alllsbrooks would have been allowed to proceed through the checkpoint with a handgun in his bag. Police said they were continuing to investigate and were “looking into what happened before, during and after those four minutes.”

Capitol Police initiated a brief lockdown during the incident, telling other workers in the building to stay inside their offices, lock doors and windows and, if they were in a public space, to find a place to hide or seek cover. The lockdown was lifted about nine minutes later.

Allsbrooks was arrested on a charge of carrying a pistol without a license. It wasn’t immediately clear whether he had an attorney who could comment on the allegations.

The arrest comes as Capitol Police track an unprecedented number of threats against lawmakers, thousands more than just a few years ago. In an interview with The Associated Press in September, Capitol Police Chief Tom Manger predicted authorities would respond to close to 9,000 threats against members of Congress in 2021 — more than 4,100 had been reported from January to March.