LOS ANGELES (AP) — Hundreds of thousands of summer travelers suffered through another day of delayed and canceled flights Wednesday, a troubling glimpse into what could happen over the long July 4 holiday weekend as airlines struggle to keep up with surging numbers of passengers.

By early evening on the East Coast, nearly 5,800 U.S. flights had been delayed and 1,000 more were canceled, according to FlightAware.

United Airlines, which depends on the airport in Newark, New Jersey, canceled the most flights among U.S. airlines for a fifth straight day.

“We’re beginning to see improvement across our operation,” United said in a statement Wednesday night. “As our operation improves in the days ahead, we will be on track to restore our operation for the holiday weekend.”

The worst disruptions continued to be along the East Coast, which has been pummeled by thunderstorms this week. The Federal Aviation Administration temporarily held up Boston-bound flights on Wednesday. It stopped flights to all three major airports in the New York City area and two near Washington, D.C., at times Tuesday.

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

Travel has picked up steadily every year since bottoming out during the pandemic. For the past week, about 2.6 million people a day on average have been flying in the United States, about 2% more than in the same period during pre-pandemic 2019, according to Transportation Security Administration figures.

The number of air travelers could set a pandemic-era record over the holiday weekend. The FAA expects Thursday to be the busiest, with more than 52,500 total flights.

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.

Tia Hudson was back at Newark Liberty International Airport for the fourth straight day, trying to catch a United Airlines flight home to Louisiana.

“My flight has been canceled like five times now. I slept at the airport two nights, I booked two hotels, I spent over $700 since I’ve been here and they said they’re not going to reimburse me because it’s weather-related,” she said. “It’s not weather-related. It’s a shortage of pilots and attendants.”

Hudson missed her mother’s wedding and caused her mother to skip her own honeymoon to pick up Tia at the airport near Dallas – only for the flight to be canceled.

On top of that, Hudson’s bags were lost.

“I just want to get away from this airport, but they say nobody is leaving until Saturday,” she said.

At Logan Airport in Boston, pharmaceutical company manager Rui Loureiro had to scrap plans to spend the rest of the week meeting clients on the West Coast when his flight to San Francisco was canceled. United told him the soonest he could get on another flight was Friday, and didn’t offer to pay for a hotel room. He plans to fly home to Portugal instead – or at least give it a try.

“I am a little bit stressed, disappointed,” Loureiro said. “People were waiting for me. We had things arranged to do. Now I have to go back and rebook everything and come again another time.”

By early Wednesday evening, Chicago-based United had canceled more than 550 flights — one-fifth of its schedule — and 1,200 others were late. The airline vowed to reduce cancellations — and added it reduced the backlog of stranded bags in Newark by 30% since Tuesday. New York-based JetBlue canceled nearly one in 10 flights.

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 on Monday night.

The Association of Flight Attendants, which represents United’s cabin crews, said United was contributing to the situation. The union said employees were waiting three hours or longer when calling a crew-scheduling center for assignments because of “limited telephone lines and personnel.”

However, the FAA signaled earlier this year that it could struggle to keep flights moving in New York, the busiest airspace in the nation. Facing a severe shortage of air traffic controllers at a key facility on Long Island, the FAA persuaded airlines to trim their summer schedules to avoid overloading the system.

Sen. Ted Cruz, a Republican from Texas, where American and Southwest are based, took the airlines’ side in a series of tweets about delays at the New York City area’s three big airports.

“Staffing at FAA’s air traffic facilities in NY is at 54%,” Cruz tweeted. “Yet (the Transportation Department) is blaming weather for delays. Nonsense.”

A Transportation Department spokesperson said the agency is working with airlines to fix things, but the combination of high traffic and bad weather reduces the rate at which planes can take off and land safely, leading to delays and canceled flights.

The FAA 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.

Last week, Buttigieg issued a new warning to airlines, telling them that planes that aren’t outfitted with new radio altimeters — devices that measure the height of a plane above the ground — won’t be allowed to operate in limited visibility starting this Saturday because of potential interference from new 5G wireless service.

American, United, Southwest, Alaska and Frontier say all of their planes have been retrofitted, but Delta Air Lines still has about 190 planes waiting to be updated because its supplier doesn’t have enough altimeters. Delta said it will schedule those planes to avoid landing where the weather might be bad to limit disruptions.

Smaller airlines that operate regional flights could also be affected by the radio interference issue, as could flights operated to the United States by foreign carriers.