Originally Published: 29 JUN 23 07:49 ET

Updated: 29 JUN 23 09:01 ET

By Chris Isidore, CNN

New York (CNN) — The problems with flight delays and cancellations that have plagued US travelers this week are mostly over — except at struggling United Airlines.

Tracking service FlightAware shows that there are 383 flights to, from or within the United States that have been canceled as of 9 am ET Thursday and another 963 delayed. For canceled flights, the problem is clearly focused on United, with 318 of those cancellations. That means 11% of United flights scheduled for Thursday were already canceled.

The next greatest number of canceled flights for a US airline is only 15 for JetBlue. That’s only 1% of its flights.

Even the problems at United early Thursday would mark an improvement from Tuesday and Wednesday, when the airline had to cancel more than 700 flights each day, or more than a quarter of its schedule. But United could see additional delays and cancellations as Thursday progresses. The FAA warned that a ground stop was likely Thursday afternoon at the two Chicago airports due to severe thunderstorms expected into the area. That would mean that not only are planes on the ground there prevented from taking off, but flights due to land in Chicago would have to delay departures at their origin airport.

Chicago is United’s home base, with one of its largest hubs at O’Hare Airport. Groundstops are also possible for Denver and San Francisco and parts of Texas, also locations of United hubs.

But beyond the bad weather there are signs that the problems at United are greatly human made.

Earlier this week United CEO Scott Kirby blamed much of the problems with the airline’s service meltdown this week on the Federal Aviation Administration and a lack of adequate staffing at air traffic control centers, particularly in the New York area, where United operates a major hub at Newark airport.

“The FAA frankly failed us this weekend,” said Kirby in an internal company memo to United staff shared with CNN.

He said the FAA reduced arrival rates at Newark by 40% and departure rates by 75% was “almost certainly a reflection of understaffing/lower experience at the FAA.”

“It led to massive delays, cancellations, diversions, as well as crews and aircraft out of position,” he said.

But Transportation Secretary Pete Buttigieg, who oversees the FAA, pushed back on that criticism of his agency Wednesday evening when appearing on CNN Primetime.

“Look, United Airlines has some internal issues they need to work through. They’ve really been struggling this week, even relative to other US airlines,” he told CNN’s Kaitlan Collins. While he agreed there needs to be more air traffic controllers, he said “I want to be very clear, air traffic control issues are not the number one issue causing cancellations and delays. They’re not even the number two issue. All the data, including industry’s own data is very clear on that.”

And United management’s handling of the service meltdown was also attacked by the two major unions representing its pilots and flight attendants.

“United’s travel disruptions this week stem from one source; company senior management’s inadequate planning an insufficient investment in the airline infrastructure,” said Captain Garth Thompson, head of the Air Line Pilots Association unit at United. “Our pilots agree with our passengers that this lack of foresight and disregard of warning signs is unacceptable. It’s time for United leadership to change their thinking and invest in its labor, staff support, and facilities with updated contracts instead of ensuring our CEO has the highest salary.”

United did not have an immediate response to the comments from Buttigieg and the pilots union.

Earlier in the week the Association of Flight Attendants also said that the problems were not just caused by weather or inadequate air traffic control staffing.

“The levels of frustration are high and it feels as if there is no solution in sight – especially for those who have been on duty for extended periods of time,” the Association of Flight Attendants said in a memo shared with CNN Tuesday.

The union message sent Monday to members cites hours-long hold times with company crew schedulers, hinting at problems beyond air traffic control staffing.

“Because hold times are currently in excess of 3 hours as crew scheduling works to update crews in chronological order, we are working to find solutions to relieve some of the backlog,” the memo said.

Asked about the flight attendants’ complaints, United responded by sating that “making sure our flight attendants can reach us quickly is a top priority. We have deployed all available resources to catch up on call volume, including increasing staffing in crew scheduling and mandatory overtime on the scheduling team. We also have ways flight attendants can check in electronically for trips and schedule changes.”