Reports of staffing shortages at eight U.S. air traffic control centers reemerged yesterday and continued into this morning, as controllers are working without pay during the ongoing government shutdown.

According to the Federal Aviation Administration’s (FAA) operations log, air traffic control centers in Newark (New Jersey) and Austin (Texas), as well as facilities managing flights arriving from or departing to Newark and Las Vegas, were understaffed.

En-route centers that typically handle high-altitude flights — located in Denver, Jacksonville (Florida), and Fort Worth (Texas) — also reported staff shortages. The issues in Las Vegas and Austin led to ground stops, forcing some flights bound for those cities to wait for takeoff clearance.

CNN’s review of previous FAA operational data shows that since the start of the government shutdown, 188 cases of staffing shortages have been reported — compared to just 45 during the same period last year.

The cause of the shortages remains unclear, but Transportation Secretary Sean Duffy said last week that some employees have taken unscheduled leave in protest of not being paid during the shutdown.

Not all instances of understaffing result in delays, as flights can sometimes be rerouted through better-staffed regions. However, in some cases, flights must be slowed down to maintain safety with limited air traffic control personnel.