Police are sending negotiators to engage with a man in the truck making the threat, one of the sources said.
Two nearby office buildings for the Library of Congress and US House are being evacuated due to a suspicious vehicle in the vicinity, according to alerts sent to staffers viewed by CNN.
Staffers in the Cannon and Jefferson buildings on Thursday were alerted by email to remain calm and relocate. Staffers in the Jefferson building were told to not use the exits on the west side of the building.
The Senate and House are not in session, and most lawmakers are not currently in their offices.
This story is breaking and will be updated.