On Sunday, CNN reported that the CDC held a video conference with approximately 800 staff members to discuss the traumatic event and allow them to ask questions. Staff said they felt like "sitting ducks" as bullets whizzed just over their cubicle walls and they dove for the floor.

Some public health workers say the shooting that killed a police officer at the CDC campus shouldn't be surprising in the face of ongoing misinformation and animosity about immunizations.

In at least six internal calls on Monday, CDC employees told leaders they worried that the agency wasn’t prepared for future threats, according to people on the calls. The potential for unauthorized people getting on campus or slow warnings about possible attacks were among the concerns shared with leaders.

The health secretary is expected to visit the agency's Atlanta headquarters later Monday to speak with employees after last week's shooting.