The Pentagon was briefly locked down and partially evacuated on Thursday after a hazardous materials alert was triggered by a sensor system later believed to have malfunctioned.

Axar.az reports, citing CNN, the incident was caused by a false alarm indicating a possible anthrax detection, prompting an immediate security response and shelter-in-place orders across parts of the building.

As of early afternoon, the lockdown had not yet been fully lifted.

Pentagon spokesman Sean Parnell said the building’s systems detected an air-quality issue requiring precautionary measures.

“The Department is executing standard protection protocols, including a shelter-in-place order for the affected area,” Parnell said. “Response teams are in place and ready to support building occupants.”

He added, “Systems within the Pentagon had detected an air quality issue necessitating precautionary measures until we determine its significance.”

Internal security guidance stated that additional testing was required and could take time.

Arlington County Fire Department hazardous materials teams were deployed to assist Pentagon responders. Officials said personnel inside the building were seen wearing protective gear, including gas masks.

Experts noted that large federal facilities often use continuous air monitoring systems capable of detecting potential biological threats, though false alarms can occur and require rapid precautionary responses.