The U.S. Capitol Building was placed in a brief lockdown in response to what announcements on the grounds described as an "external security threat," Fox News has learned.
The threat appears to have been linked to an outdoor fire at a homeless encampment that has since been extinguished.
The encampment was located approximately one mile away, where a plume of smoke created security concerns. Washington, D.C. Fire and EMS said the fire "accounts for smoke that many have seen."
People on Capitol grounds were told to remain inside shortly before 10:30 a.m. Monday, when people were urged to stay away from exterior-facing doors and windows.
The threat was cleared before 11 a.m., with the U.S. Secret Service saying in a subsequent tweet that authorities "responded to a small fire," which prompted the "temporary shutdown" of the U.S. Capitol complex.
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In an alert sent to congressional staffers that was obtained by Fox News, U.S. Capitol Police said: "Due to an external security threat under the bridge on I-295 at First and F Streets SE, staff and other personnel are directed to avoid coming to the Capitol Complex area until further notice. All personnel currently on the Capitol Complex are advised to stay indoors and away from windows and doors."
Before given the all-clear, U.S. Capitol Police and other on-the-ground law enforcement could be seen hurrying to investigate.
The U.S. Capitol Building and state capitals around the country are ramping up security in the days leading up to Wednesday's Inauguration of President-elect Joe Biden.
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An Inauguration rehearsal was underway when the threat was first reported, which prompted the rehearsal to be evacuated. Biden did not attend the rehearsal.
Tensions on Capitol Hill are high in the wake of the events of Jan. 6, when a violent mob of hundreds of apparent Trump supporters stormed the federal building.
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Five people, including a U.S. Capitol Police officer, died during or in connection with the siege.
Fox News' Edward Lawrence and Rich Edson contributed to this report.