Firefighters are investigating a hazardous materials incident at the Pentagon, Arlington County fire and rescue officials say in a post on X.
The building was under lockdown with people told to leave several floors, CNN reported, citing unidentified sources.
It said emergency responders were wearing full gas masks and chemical protection suits.
"The Pentagon has sophisticated systems to ensure the safety of the building and its occupants. Those systems have detected an air quality issue necessitating precautionary measures until we determine its significance," Pentagon spokesman Sean Parnell said in an email.
"The Department is executing standard protection protocols, including a shelter-in-place order for the affected area. Response teams are in place and ready to support building occupants."
Floors two through five were locked down as well as some corridors, CNN reported.
The five-sided Pentagon building is one of the world's largest office buildings.