Close to 100 flights in and out of Denver International Airport were canceled on Tuesday, stranding hundreds of passengers who had to sleep on cots. 

The bulk of canceled flights were by United Airlines.

As of Tuesday afternoon, 62 flights to Denver were canceled, while 34 flights from DIA didn’t occur.

It’s not immediately clear why so many United flights were canceled, but United’s CEO blamed the FAA’s staffing shortages in an email to employees on Monday evening. 

By 5 p.m. Tuesday, some 500 United Airlines flights had been canceled in the U.S., the most by any airline, according to tracking website FlightAware. 

The cancellations had begun over the weekend and irate customers vented their frustrations on social media.

United customers posted photos of hours-long phone calls on hold with United customer service and piles of boxes for the cots they slept on overnight. 

Robert Wark, who was traveling out of Newark on Sunday, said flights were canceled but not one United employee could tell people what’s happening or when folks could retrieve their bags.

“What a nightmare!” Wark said.

#unitedAIRLINES has cancelled a ton of flights at Newark with not one employee able to tell us what’s happening, or when we can retrieve our bags. What a nightmare! Never flying #unitedAIRLINES again.

— Robert Wark (@WarkhardR) June 25, 2023

Another passenger said Monday that her flight had been canceled from Los Angeles to Newark to Dublin, Ireland.

“We have 71 teens and chaperones that have been waiting patiently at airport since 5AM with no resolution,” she said. “What’s the deal? They’ve been prepping for their choir festivals for a long time. Help!”

Hi there. We’re truly sorry to hear about your group’s cancelation. When you get the chance, please DM your confirmation number so we can take a closer look. ^RZ https://t.co/Y6hG6u3i8R

— United Airlines (@united) June 26, 2023

United’s social media team apologized and sought to assist passengers who had complained on Twitter.

“We’re truly sorry to hear about your group’s cancelation,” United told the passenger who was traveling with 71 teens. “When you get the chance, please DM your confirmation number so we can take a closer look.”

In a message to employees, United CEO Scott Kirby said the Federal Aviation Administration “failed us.”  

FAA staffing issues in the midst of East Coast weather challenges are at the core of United’s operational issues, Scott said. Over 150,000 customers are estimated to have been impacted last weekend by “FAA staffing issues and their ability to manage traffic.”

Scott said the FAA’s staffing issues “led to massive delays, cancellations, diversions, as well as crews and aircraft out of position.”  

“I’m also frustrated that the FAA frankly failed us this weekend,” he added.

In his letter, Scott said the FAA’s current leadership is not to blame. Rather, he said, understaffing has been “building up for a long time before (the current leadership members) were in charge.”

Scott said he plans to get in touch with the FAA and U.S. Department of Transportation to discuss steps to prevent Tuesday’s cancellations from happening again. 

“While I’m frustrated that the FAA is letting us and our customers down, I’m encouraged at everything all of you are doing to manage the best we can,” Scott said. “At United, our team will continue to do great things to take care of customers no matter what happens outside of our control.”

FAA spokesperson Emma Duncan said there were no East Coast staffing issues on Monday or Tuesday. 

The FAA “will always collaborate with anyone seriously willing to join us to solve a problem,” Duncan added. 

A DIA spokesperson said airport authorities do not have information about the reason for United’s cancellations or how many people at the airport were affected. 

DIA officials urged anyone affected by the cancellations to reach out to United. 

Denver Gazette reporter Carol McKinley contributed to this report.