WASHINGTON (AP) — Four airports serving Washington, D.C., Baltimore and Richmond, Virginia, halted all flights on Friday evening for over an hour because of a strong chemical smell that was impeding air traffic controllers, according to the Federal Aviation Administration.

The ground stop affected Ronald Reagan Washington National Airport, Washington Dulles International Airport, Baltimore-Washington International Airport and Richmond International Airport, FAA Secretary Sean Duffy announced on social media Friday. The declaration caused flight delays to soar to roughly two hours across some of the busiest airports in the country.

Flights began to leave the airports after 7 p.m. ET on Friday, but the ground stop — which prevents planes from landing at an airport — remained in place.

The smell was coming from Potomac TRACON, Duffy wrote, referring to a terminal radar approach control facility that manages air traffic for the Washington, D.C., Baltimore, Richmond, Virginia, and the Richmond-Charlottesville areas, according to the FAA website.

A spokesperson for the federal agency didn’t respond to an emailed question clarifying how the smell was affecting traffic controllers on Friday evening.

Between 25% and one-third of all flights departing from the four airports affected were delayed after the ground stop.