Red mercury is reportedly a substance used to create nuclear bombs, but its existence has not been documented.
Brown said Homeland Security is en route to the location.
Parts of Peachtree Street, Peachtree Center Avenue and Courtland Street have been closed and officers are redirecting traffic.
A hazmat team, bomb squad and fire trucks were seen near the investigation.
FIRE under a bridge in downtown Atlanta with overpass collapse. Click link below pic for video.