A week into the government shutdown — and days before the start of a holiday weekend — travelers at some U.S. airports have already been feeling the pain of flight delays.

There have been more than 12,000 flights delayed at U.S. airports from Monday through Wednesday evening, according to the flight tracking service FlightAware . But it’s not clear how many can be blamed on the shutdown.

Transportation Secretary Sean P. Duffy has warned that delays and cancellations could result from the lapse in funding. Air traffic controllers and Transportation Security Administration officers are supposed to work without knowing when they will be paid; Duffy said this week there had been a “slight tick-up” in controllers calling in sick.

“You realize when these types of things happen how rickety t

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