DALLAS (AP) — Air travelers endured another wave of flight disruptions Thursday despite better weather along much of the East Coast, while United Airlines continued to account for the majority of canceled flights nationwide.

United vowed to get back on track over the July 4 holiday weekend, when the number of air travelers could set a pandemic-era record.

Hundreds of thousands of people have had travel plans thrown in the air after a wave of storms raked the Northeast over the past few days and frustrations are running high.

Airports in Chicago, Denver and Newark, New Jersey — all hubs for United — were seeing the most delays on Thursday, according to FlightAware.

By early evening on the East Coast, United had canceled more than 400 flights, the bulk of the 600-plus cancellations toted up by FlightAware. The Chicago carrier was poised to lead all U.S. airlines in cancellations for a sixth straight day.

United CEO Scott Kirby has blamed the airline’s struggles in Newark on a shortage of air traffic controllers in the New York City area. Transportation Secretary Pete Buttigieg pushed back against the criticism while conceding that a key Federal Aviation Administration facility in New York is severely understaffed.

“United Airlines has some internal issues they need to work through. They have really been struggling this week, even relative to other U.S. airlines,” Buttigieg told CNN. “But where we do agree is that there need to be more resources for air traffic control.”

The FAA plans to hire 3,300 controllers over two years, but they won’t be ready to help this summer, much less this weekend.

The leader of United’s union pilots — who are locked in difficult contract negotiations — blamed management for the disruptions, saying the company failed to upgrade a crew-scheduling system.

“While Scott Kirby attempts to deflect blame on the FAA, weather and everything in between, further flight delays are a direct result of poor planning by United Airlines executives,” Garth Thompson said.

United is offering triple pay to flight attendants who are scheduled off this weekend but agree to pick up extra flights, according to their union. The Association of Flight Attendants also said crews calling in for assignments have been put on hold for three hours or longer.

“The airline actually ‘lost’ crews in the system for days on end because there was such a significant breakdown in running the operation,” said Ken Diaz, president of the flight attendants’ group at United. He said the company scheduled summer flights “to the max” even knowing about air traffic control limits in the Northeast.

United said it was getting a handle on its problems.

“We’re seeing continued meaningful improvement today after an overnight effort to further repair schedules and match separated crews with aircraft,” the airline said in a midday statement. “As the recovery progresses, delays and cancellations will continue to decline as we head into what we expect to be a very busy holiday weekend.”

Because planes are packed for the summer, it is hard for airlines to rebook customers when flights are canceled – there aren’t many empty seats.

Ariana Duran of Orlando, Florida, said JetBlue rebooked her for seven days later when her flight home from Newark was canceled this week. So she got creative.

Her boyfriend spotted a $1,200 seat on another airline, but it was gone before they could buy it. She looked into Amtrak. Duran, who does marketing for an insurance company, wound up paying about $640 for a one-way ticket to Orlando — with a stop in the Florida Keys — on United Express and Silver Airways.

Duran didn’t care when they told her it would be a very small plane.

“Just put me on a boat at this point,” she said.

Sonia Hendrix, who runs a public-relations firm in New York, took four days to get home from a business trip to Colorado. She was stranded one night in Atlanta and two in Orlando, when connecting flights were canceled. She woke up some mornings not knowing where she was.

Hendrix said a Delta agent threatened to cancel her reservation when she complained about being offered only a $50 voucher for her trip troubles. On the other hand, her pilot, “Captain Dan,” waited in the airport and helped passengers after the flight was canceled.

“This is not all Delta’s fault. Their pilots and flight crews were working very hard,” she said. “I blame the FAA. I blame Buttigieg for sitting on his hands” and not staffing up air traffic control centers sooner.

Hendrix said she lost working time and spent $700 out of pocket. She is so worried it could happen again that she is reconsidering a business trip next month to Los Angeles.

The FAA said Thursday would be the busiest day of the holiday stretch by number of flights. The Transportation Security Administration said it expected to screen the most travelers on Friday — a predicted 2.82 million people.

Scattered showers and thunderstorms may arrive later Thursday in the Northeast, and storms were also forecast farther south along the East Coast through Saturday. The West is under threat of unstable weather for the next several days.

Along with big crowds and storms, a technology issue could add to travelers’ difficulties. Federal officials say some airline planes may be unable to fly in bad weather starting Saturday because of possible interference from new 5G wireless service.

American, United, Southwest, Alaska and Frontier say all their planes have been retrofitted with new radio altimeters — those are devices that measure the plane’s height above the ground — and they do not expect disruptions due to 5G service.

However, Delta Air Lines has about 190 planes in its fleet of more than 900 that have not been updated because it can’t get enough altimeters from its supplier. Delta says it will schedule those planes to avoid landing in poor visibility while it works to upgrade them through the summer.

The issue affects several types of single-aisle planes that Delta uses on routes within the United States, including all its Airbus A220s and most of its Airbus A319 and A320 jets.

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.

By early evening in the East, about 5,300 flights had been delayed, down from an average of 8,000 a day over the first three days of the week.