Minute by Minute: Police investigate suspected explosives in a truck near the US Capitol.

(CNN) – House buildings are being evacuated after police responded to a possible explosive device in a truck near the Library of Congress, according to law enforcement sources. Police reported that they are treating the incident as a “bomb threat investigation.”

Police are sending a negotiator to speak to a man inside the truck, one of the sources said.

The US Capitol Police said via tweet that they were responding to the incident of a suspicious vehicle nearby. Congressional staff have been alerted to take refuge in their offices, according to messages seen by CNN.

MEDIA ALERT: This is an active bomb threat investigation. The staging area for journalists covering this situation is at Constitution and First Street, NW for your safety.

Please continue to avoid the area around the Library of Congress. pic.twitter.com/jTNVaBmVwR

– US Capitol Police (@CapitolPolice) August 19, 2021

Minutes later, in another tweet, the Capitol Police reported that they are calling this an “active bomb threat investigation.”

The FBI said in a statement to CNN that the Washington Office’s National Capital Response Squad is “responding with our partners to reports of a suspicious vehicle near the Library of Congress.”

Two House of Representatives office buildings are being evacuated due to the presence of the vehicle, according to alerts sent to congressional staff seen by CNN.

Employees at the Cannon and Jefferson buildings were alerted by email to remain calm and relocate. Employees at the Jefferson Building were told not to use the exits on the west side of the building.

The Senate and House of Representatives are not in session, and most legislators are not in their offices.