The Pentagon has lifted “precautionary safety measures” that it implemented Thursday morning, including a shelter-in-place order for parts of the complex, after determining that the reported “hazardous materials incident” that prompted the response was a false alarm.

“Earlier this morning, Pentagon occupants were notified of a potential air quality issue, prompting immediate precautionary safety measures and evaluation. Subsequent testing confirmed no hazard exists, and normal operations have resumed,” Pentagon spokesman Sean Parnell wrote on X at 1:31 p.m. ET, just under three hours after emergency response units reported that they were responding to the incident. “We express our sincere appreciation to the first responders for their swift actions to ensure the safety of all personnel.”

Earlier on Thursday, Parnell told TIME in a statement that the department was following protocols and enforcing a shelter-in-place order, among other measures, in response to the detection of the “air quality issue.”

"The Pentagon has sophisticated systems to ensure the safety of the building and its occupants. Those systems have detected an air quality issue necessitating precautionary measures until we determine its significance,” Parnell said. “The Department is executing standard protection protocols, including a shelter-in-place order for the affected area. Response teams are in place and ready to support building occupants." 

The Arlington County Fire Department (ACFD) posted on X at 10:41 a.m. that it was responding to the incident. 

“ACFD units, including our Hazardous Materials Team, are currently operating at the Pentagon,” the department wrote, noting that they were working in conjunction with the Pentagon Force Protection Agency (PFPA) Hazmat Team in response to a “hazardous materials incident.”

At 12:24 p.m., the PFPA wrote on X that the “shelter in place continues in affected areas in the Pentagon until all clear is given.”

Sources familiar with the matter told CNN Thursday morning that multiple floors and corridors at the Pentagon had been locked down, including floors two through five in the fourth through the seventh corridors. Others were being evacuated and police in the building were wearing full chemical protective gear and gas masks, sources told the outlet. 

CNN also reported that the Pentagon’s security team issued a message that said additional testing was needed following the detection of the “air quality issue.”  

“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,” said the message, according to CNN.