Odd day to be threatening something
There’s a 99.9% chance this is nothing but given recent events, it’s notable. However it’s an odd day to be phoning in a threat because it’s a holiday in the US and many (most?) politicians won’t be there.
An alert told people to stay away from windows. The inauguration rehearsal was also evacuated.
Update: There are a reports of a fire somewhat close to the Capitol and that might have been what caused the alarm. The fire has been extinguished.