Capitol police evacuated the U.S. Capitol on Tuesday, but the reason for the evacuation was not immediately clear.
Initial reports suggested the operation was limited to the visitors center, but police told NBC that people in the main Capitol facility were also required to evacuate.
An audible alarm was heard at the facilities, according to the Senate sergeant at arms Twitter account.
Capitol Police said in an emailed statement that they were investigating the alarm, but they did not immediately see any signs of smoke or fire.
Just before 1:20 p.m. ET staff were allowed to reenter the U.S. Capitol.
—NBC contributed to this report.