Police and firefighters have been called to the scene at 3rd St and Madison Dr NW.
The man claimed to have anthrax, law enforcement sources told ABC News. Although initial field tests for the substance were negative.
According to police, the man said he was exposed to anthrax at a farm in Virginia. He collected some of the substance and brought it to the National Mall.
During the ordeal, police drove a robot around the truck:
Robot makes its way around truck on National Mall pic.twitter.com/nncHVU8FE6— Jim Stevenson (@VOAStevenson) May 17, 2016
Authorities are interviewing the suspect and clearing the scene of any other potential hazards.
Police have blocked off the area around the incident and are asking the public to stay away from the area.