Travellers south of the border got hit with delays at US airports again Wednesday, an ominous sign heading into the long July 4 holiday weekend, which is shaping up as the biggest test yet for airlines that are struggling to keep up with surging numbers of passengers.

By early afternoon on the East Coast, according to FlightAware, nearly 3,000 US flights had been delayed and more than 800 cancelled.

The worst disruptions continued to be along the East Coast, which has been pummeled by thunderstorms this week. The Federal Aviation Administration temporarily grounded flights going to all three major airports in the New York City area Tuesday night after holding up flights heading to Reagan Washington National and Baltimore-Washington airports near the nation’s capital earlier in the day.

Huge crowds, bad weather, the inability of some airline crews to reach their scheduling offices – even a Delta jet that made a belly landing in Charlotte, North Carolina – have all contributed to the mess.

And it could be just the storm before the storm: The FAA predicted that Thursday would be the heaviest travel day over the July 4 holiday period. On top of that, some airline planes may be unable to fly in bad weather starting this weekend because of possible interference with 5G wireless service.

United Airlines, which has a major hub operation in Newark, New Jersey, was again faring the worst early Wednesday. It cancelled about 400 flights or 13% of its schedule by early afternoon. New York’s JetBlue cancelled 9% of its flights.

About 6,500 flights were delayed and about 1,900 cancelled on the East Coast Tuesday. United cancelled about 500 flights or 18% of its schedule, and JetBlue cancelled 16% of its flights, according to FlightAware.

People whose travel plans were disrupted took to social media to vent against the airlines. Typically, some swore they would never fly again on whichever airline had done them wrong.

If large numbers of passengers are stranded or delayed this weekend, expect federal officials and the airlines to blame each other for the mess.

US Transportation Secretary Pete Buttigieg, whose department includes the FAA, has been beating up on the airlines for more than a year. He has accused them of failing to live up to reasonable standards of customer service and suggested that they are scheduling more flights than they can handle.

The airlines are punching back.

United Airlines CEO Scott Kirby blamed a shortage of federal air traffic controllers for massive disruptions last weekend at its Newark hub.

“We estimate that over 150,000 customers on United alone were impacted this weekend because of FAA staffing issues and their ability to manage traffic,” Kirby wrote in a memo to employees Tuesday night.

United could be contributing to its struggles. The Association of Flight Attendants, which represents the airline’s cabin crews, said it complained about wait times of more than three hours for workers who called a crew scheduling centre that had “limited telephone lines and personnel.” The union told flight attendants near the end of their shifts to tell supervisors and find a hotel room.

The FAA has admitted that it is understaffed at key facilities including one in the New York City region. It is training about 3,000 new air traffic controllers, but most of them won’t be ready anytime soon. Last week, the Transportation Department’s inspector general said in a report that the FAA has made only “limited efforts” to adequately staff critical air traffic control centres and lacks a plan to tackle the problem.