The Capitol was placed under a lockdown for about 40 minutes on Friday morning, when police falsely believed that an employee had slipped a gun into the building.

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Police ordered a lockdown of the building and the adjacent visitor’s center when they spotted something in an employee’s backpack “that required further investigation,” U.S. Capitol Police said in a statement. The employee was heading to the Capitol through an underground tunnel from the nearby Rayburn House Office Building and appeared to have gone through X-ray machines and security guards at between the two buildings.

“Out of an abundance of caution, the Capitol was placed on lockdown to keep everyone safe until the circumstances could be verified,” the police said.

The lockdown was lifted at about 9:40 a.m., when police determined that the object was not a threat.

Lawmakers confirmed to The Hill that police were concerned about the presence of a firearm at the Capitol.

Capitol staffers were told during the lockdown to go into an office room and “close, lock and stay away from external doors and windows.” People were prevented from entering or leaving the Capitol Building and the visitor’s center, which is populated with tourists throughout the summer.

The House briefly gaveled into session and then gaveled out during the lockdown, as lawmakers sheltered in place in the chamber.

The closure on Friday morning came as lawmakers and the public were still grappling with news that five police officers were killed in Dallas hours before.

In March, the Capitol scrambled into action when a 66-year-old Tennessee man allegedly approached the visitor’s center wielding a firearm and pointing it at officers. The man, Larry R. Dawson, was shot by police and later taken into custody and charged with assault.

—Jonathan Easley and Scott Wong contributed. Last updated at 1:26 p.m.