Maybe the plane’s air conditioning hadn’t come on yet, he thought briefly before slipping on his headphones and scrolling through the movie selections.

But the situation deteriorated, with some passengers and crew falling ill and ending with a canceled flight that joined the litany of airline horror stories across social media. The Department of Transportation is investigating the incident, U.S. Transportation Secretary Pete Buttigieg told Reuters news agency.

“I want to know how it was possible for passengers to be left in triple-digit heat onboard an aircraft for that long,” Buttigieg said, calling the issue “infuriating” and “shocking.”

The flight disruption, which spanned several hours, illustrates how the heat wave is making life miserable for Americans, increasing the risk of wildfires in California and prompting health warnings in places like Phoenix, where temperatures measured above 110 degrees for 21 straight days, setting a new city record. Previous heat waves have delayed flights, making commercial flying even more challenging for harried travelers.

Harry Reid International Airport in Las Vegas saw temperatures of over 100 degrees on Monday, according to the National Weather Service. Sin City is under an excessive heat warning through 8 p.m. Saturday.

The oppressive heat wave is expected to last through August.

On the East Coast, the Federal Aviation Administration warned this week that thunderstorms could lead to flight delays in Washington, Chicago and Philadelphia, among other cities.

“We apologize for the experience our customers had on flight 555 from Las Vegas to Atlanta on July 17, which ultimately resulted in a flight cancellation,” a Delta spokesperson said. “Delta teams are looking into the circumstances that led to uncomfortable temperatures inside the cabin and we appreciate the efforts of our people and first responders at Harry Reid International.”

Passengers had the opportunity to exit the plane each time it returned to the gate that day, a Delta spokesperson said.

Clarke, 49, had arrived at the airport around noon for his 1:47 p.m. flight back to Atlanta. As he waited at the gate, a Delta agent announced that the flight was delayed because the staff was down a crew member, he said.

When passengers eventually boarded, a flight attendant made an announcement for passengers to open their vents as much as possible to keep the air flowing, he said. A few young children were seated beside him, whose vents he helped open.

As the American Southwest experiences an unbearable heatwave, A delta flight from Las Vegas to Atlanta is canceled due to rising temperatures inside the plane.

The plane’s door was closed. The cabin grew uncomfortably warmer. “I thought it was a normal flight where we’d take off and it will cool down,” he said.

According to Clarke, about 30 minutes passed when crew said they needed volunteers to exit the plane because it exceeded its weight limit. A spokesperson for Delta confirmed that customers were asked to give up their seats and attributed the issue to the way metal, an aircraft’s primary material, reacts to extreme temperatures. Some passengers stepped off, and the door once again closed behind them. The Delta spokesperson declined to comment on the number of passengers who left the plane.

As the plane taxied the runway, the pilot announced they were near the front for takeoff, Clarke recalled. But what he estimated was nearly 90 minutes later, an announcement said that several more planes moved in front of them, he said.

The cabin was stifling.

Passengers around Clarke were fanning themselves with the airline brochures. Their clothes stuck to their skin. He could see one of the flight attendants jumping on and off the phone.

“Next thing you know, a few of the crew start moving fast up and down the aisle,” he said. Word also sped through the cabin: A passenger had passed out. Another passenger was throwing up.

Two passengers were brought to the front of the plane and given oxygen, Clarke, who was seated nearby, said.

Other passengers described the ordeal in similar detail in social media posts. April Love, a publicist based in Atlanta, shared a photo on her Facebook account of staff treating passengers with oxygen. “Medical emergency!!” she wrote across the photo in bold, black text with a siren emoji.

“It was super hot, so I stood up because it was super hot,” Love said in an interview. “I was a little bit afraid … and unnerved.”

The flight returned to the gate and paramedics boarded the aircraft. Krista Garvin, a field producer for Fox, posted a video on Twitter of a yellow ambulance with blinking lights parked beside the plane. In the video, the captain can be heard over the intercom apologizing for the “situation being very hot in the back there.” The captain can also be heard saying that the cooling system was working the “best” it could while passengers were onboard the aircraft.

A Delta spokesperson said that the aircraft’s air conditioning and cooling system was functioning. Passengers were given the option to exit the plane when it returned to the gate, but the same announcement also said there were no other flights to Atlanta, passengers said. Clarke said he needed to get back to Atlanta and chose to stay on.

Delta declined to say how many total passengers received medical attention, but the airline said one passenger and one flight attendant were transported to a local hospital for heat-related illnesses. Garvin posted that she’d seen three people “wheeled out.”

The cabin felt like a “hot box,” Clarke said.

The ordeal unfolded over about three hours, according to passengers and to Delta. The plane returned to the gate once again, and all passengers were deplaned so the aircraft could “cool down,” Clarke said. His shirt collar was soaked. A wet spot formed across his back.

“It looked like I had been in the gym,” he said. Passenger after passenger walked off the plane and glistened with sweat, he said.

Delta eventually canceled the flight, citing weather conditions, a spokesperson confirmed. An internal investigation is underway. It is unclear what temperature was recorded inside the cabin.

No other Delta flights were reported to experience similar issues that day, the spokesperson said.

Delta said that affected customers were compensated for hotel stays and provided food vouchers. (Love said the hotels did not have any rooms available, and she was forced to pay out of pocket for her lodging that evening.) Passengers will also be refunded the full price of their original ticket and receive bonus SkyMiles or an electronic travel voucher, a spokesperson said.

Love and Clarke both said they received an apology email from Delta and were informed they’d be credited 20,000 miles. Clarke was disturbed by what he said was a lack of transparency from Delta.

Clarke was rebooked on a Tuesday flight that was delayed several times before he eventually touched down in Atlanta.