CDC security guards stopped White from driving into the campus on Friday before he parked near a pharmacy across the street and opened fire from a sidewalk. The bullets pierced “blast-resistant” windows across the campus, pinning employees down during the barrage. More than 500 shell casings were recovered, the GBI said.

The U.S. CDC has tightened security following an attack Friday on its Atlanta headquarters that left a police officer and the gunman dead, including having employees work from home on Monday and removing vehicle decals showing where they work.

Officials are still investigating a shooting at the Centers for Disease Control and Prevention headquarters in Atlanta, Georgia, that killed a police officer on Aug. 8. The suspect, Patrick Joseph White,