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Pentagon police are in gas masks and full chemical gear Thursday as a hazmat team locks down floors 2 through 5 of the massive complex.

Floors 2 through 5 in corridors 4 through 7 have been locked down, and other floors have been evacuated, three sources told CNN. The Pentagon Force Protection Agency's hazardous materials response team is on scene alongside the Arlington County Fire Department.

"The Department is executing standard protection protocols, including a shelter-in-place order for the affected area," Pentagon spokesman Sean Parnell said. Systems within the Pentagon, he added, "have detected an air quality issue necessitating precautionary measures until we determine its significance."

An internal security message obtained by CNN offered a rare glimpse inside the response — and a curious instruction to rattled occupants.

"This additional testing could take one to two hours. Response teams are in place and ready to support building occupants if necessary. You may observe response personnel from multiple agencies and precautionary measures taking place in the center courtyard. Please do not interpret these activities," the message read.

Retired Col. Cedric Leighton, a former Pentagon official, flagged a particular danger for some workers on CNN.

"There are certain facilities within the Pentagon that are windowless facilities, so the only air supply that they're going to get is from the air handling system that is in the Pentagon," Leighton warned.

Arlington Fire & EMS confirmed on X that its hazardous materials team is operating at the Pentagon "during a hazardous materials incident," posting 18 minutes before the Pentagon's public acknowledgment.

Tens of thousands of people work inside the building. The full scope of how many have been locked down or evacuated remains unclear.

This is a developing story.