A false alarm at the Pentagon following an air quality warning briefly triggered a partial shelter-in-place order on Wednesday at the U.S. military’s headquarters, as first responders worked to rule out exposure to hazardous materials, officials said.

The lockdown applied to several corridors of the five-sided building in Arlington, Virginia. Employees were told to stay put while tests were carried out to determine whether a faulty sensor was to blame.

Many employees outside the areas of concern were told to leave the building as a precaution, officials said.

“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,” said chief Pentagon spokesman Sean Parnell.

“We express our sincere appreciation to the first responders for their swift actions to ensure the safety of all personnel.”

The Pentagon, one of the world’s largest office buildings, was hit during the Sept. 11, 2001 al Qaeda attacks.

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(Reporting by Phil Stewart, Doina Chiacu and Ryan Patrick Jones;Editing by David Ljunggren, Caitlin Webber and David Gregorio)