The Pentagon was evacuated and placed on lockdown Thursday following a false alarm, CNN reports, citing anonymous sources. The security measures were initiated after the building’s monitoring systems detected issues with air quality, according to Pentagon spokesperson Sean Parnell. Emergency teams and firefighters were dispatched to the scene, and police officers were observed inside the building wearing gas masks and chemical protective gear. The evacuation process can take several hours to complete. Initial concerns prompted a significant response, but authorities have since determined the incident was not a threat. The cause of the initial air quality reading is currently under investigation.