The driver of the car, located near the Capitol in the center of Washington, said a bomb was in the car.
The police saw a semblance of a detonator in the hands of a man, and negotiations are underway with him.
On Thursday afternoon, a man in a truck parked in the area of the Library of Congress and said that there was an explosive device in the car. Residents of nearby houses and office workers near the scene were evacuated.
According to a police spokesman, the identity of the suspect has been established, but his motives remain unknown.