The Defense Department on Thursday morning issued a shelter-in-place order for parts of the Pentagon and others were evacuated after an air-quality alert was activated inside the building, in Arlington, Virginia.

The situation turned out to be a false alarm, according to CNN, citing unnamed sources. 

The pentagon-shaped building, among the most secure in the world, sits on 34 acres, across the Potomac River from Washington, D.C. 

Sean Parnell, Pentagon spokesperson, said "precautionary measures" were being put in place while the matter is being assessed. 

"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," Parnell said. 

The orders are "standard protection protocols" and include a shelter-in-place order for the affected portion of the building. The order affects multiple floors of the building, and police were seen in the building wearing gas masks and full chemical protective gear.