For the second time this week, an air traffic control tower at a major U.S. airport had to be shut down after a worker tested positive for the coronavirus.

This time it was a tower at Las Vegas’ McCarran International Airport, which was temporarily shuttered for sterilization after an air traffic controller became infected with the virus, according to the FAA. Workers from nearby airports were called upon to assist with landing planes following Thursday’s closure, which resulted in numerous flight delays at McCarran, per the FAA.

The closure was only the second of its kind in aviation history. The first occurred on Tuesday when an air traffic control tower at Chicago’s Midway International Airport was temporarily closed for the same reason.