(AP) — Travelers are getting hit with delays at U.S. airports again early 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.

As of early morning, more than 900 flights have been delayed in the U.S., according to FlightAware. Cancellations topped more than 670 flights.

Cancellations and delays were most severe along the East Coast early and were growing worse rapidly. Disruptions are expected to spread West.

The delays are being compounded by thunderstorms that raked the Northeast on Tuesday. At various times, the Federal Aviation Administration held up flights bound for LaGuardia Airport in New York and Reagan Washington National and Baltimore-Washington airports near the nation’s capital.

About 6,500 flights were delayed and about 1,900 were canceled on the East Coast on Tuesday. United Airlines, with a major hub in Newark, New Jersey, canceled about 500 flights or 18% of its schedule, and JetBlue canceled 16% of its flights, according to FlightAware.

Call it the storm before the storm.

Travel has picked up steadily every year since bottoming out during the pandemic and on Tuesday, the number of people flying neared 2.4 million, up 11% from last year on the same day, according to the Transportation Security Administration.

Travel is expected to peak on Thursday with more than 52,500 total flights, likely the biggest travel day of the holiday period.

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

When their United flight in Newark was canceled Sunday night, Margo and Jason Osborne searched for other flights but couldn’t find one. Margo said a United agent wouldn’t let them retrieve their bags either – they would have to pick those up at their final destination, New Orleans — then “made me feel like an idiot” for leaving needed medical supplies in a checked bag.

They saw other people looking for unaccompanied minors and heard about stranded travelers who spent hours in line or slept at the airport.

“All these poor people are literally just sitting there at the mercy of a company who is not doing anything to help them,” Margo said in an interview. “There is zero customer service right now.”

The Osbornes rented a car — they felt lucky to snag one — and drove 10 hours through the night to Charlotte, North Carolina, to catch an American Airlines flight to Baton Rouge, Louisiana, close to home. Their luggage finally got to New Orleans on Tuesday.

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

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 center 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 centers and lacks a plan to tackle the problem.