An air-traffic controller at the Las Vegas McCarran International Airport tower tested positive for Covid-19, prompting the closing of the facility and a dramatic reduction in the rate of flights.

The controller was found to have the virus Wednesday, the Federal Aviation Administration said in a statement on Thursday. As of 9:08 a.m. local time, airlines had canceled 463 flights to and from the airport, or more than 35%, according to the flight-tracking website FlightAware.

The FAA is transferring partial control of flights to a nearby facility that oversees traffic in that region. While most airports with commercial service have towers, airliners are allowed to land without the assistance of tower controllers by following special procedures.

It was the second U.S. air-traffic tower to be affected by the spread of the coronavirus. Chicago’s Midway International Airport was shut temporarily this week after three technicians tested positive.

The agency is cleaning the Las Vegas facility to ensure no germs are present, it said in a tweet. It is also assessing how many of its controllers will have to “self-isolate” after being exposed to the person who is potentially infected, the agency said.

“The safety of our staff and the traveling public is the FAA’s top priority,” the agency said. “Our controllers, inspectors and others with critical safety or security sensitive roles are essential components of our national airspace.”