The U.S. Capitol complex temporarily locked down Monday during a rehearsal for President-elect Joe Biden’s inauguration after a fire in a homeless encampment roughly a mile away sent a plume of smoke into the air and caused security concerns in an already jittery city.

U.S. defense officials say they are worried about an insider attack or other threat from service members involved in securing President-elect Joe Biden’s inauguration, prompting the FBI to vet all of the 25,000 National Guard troops coming into Washington for the event.