The rehearsal at the Capitol Monday morning for Wednesday’s inauguration of Joe Biden was evacuated by officials who told participants “this is not a drill.” The rehearsal had been postponed one day due to security concerns.
Warnings were given over loudspeakers, “BREAKING: Smoke can be seen rising near the U.S. Capitol building. An announcement has gone out over the loud speaker telling people to stay away from doors and windows due to an external security threat”
Excerpt from AP report: “Participants were ushered from the West Front of the Capitol. Those who had gathered for a walk-through, including a military band, were directed to head indoors and moved in the direction of a secure location inside the Capitol complex. People involved in the rehearsal said security officials yelled “this is not a drill.” The U.S. Secret Service, which is in charge of security for the inauguration, said there was no threat to the public.”

Apparently the fire was at a homeless encampment under a bridge several blocks south of the Capitol.