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The National Transportation Safety Board will investigate the engine failure that caused an intense fire on a British Airways Boeing 777 as it prepared for takeoff in Las Vegas, sending 20 passengers to the hospital.

Passengers used inflatable slides to escape the blaze on Tuesday afternoon at McCarran International Airport, just before the plane was to embark on a 10-hour flight to London Gatwick.

Thick clouds of black smoke billowed from the left engine of the plane as the passengers scurried across the tarmac. Witnesses said the fire was so intense that it melted windows on the plane.

Sunrise Hospital said it treated 20 patients. The airline said Wednesday that everyone had been released. It also pledged its cooperation with the NTSB.

aftermath of british airways flight 2276 pic.twitter.com/mpHgcuiXMM — CastroBelRico (@SuaveCastro) September 9, 2015

Cockpit alarms could be heard on recordings of the pilots' mayday call to air traffic controllers.

The Boeing 777 experienced an engine failure after being cleared for takeoff at around 4:13 p.m. (7:13 p.m. ET) at McCarran International Airport, the airport said.

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Passenger Letha Dunn recalled a “tremendous explosion from behind."

"I thought, since I’m still alive, it must be just that a tire exploded. Because if an engine exploded, I probably wouldn’t be having these thoughts," she said.

Moments later, Dunn said some passengers came running from the rear of the plane, "shouting 'The plane’s on fire! We have to get off this plane!"

"We were just about to take off," London-based passenger Dominic Worthington told NBC News. He said smoke was seen on both sides of the plane, and then the evacuation chutes deployed.

There was a lot of smoke as passengers used the chutes, and they could see debris on the ground, Worthington said. At the bottom of the slide, passengers going down were bumping into each other amid the thick smoke, he said.

A total of 159 passengers were on board Flight 2276, along with three pilots and 10 cabin crew, the airline said.

Clark County Deputy Fire Chief Jon Klassen told The Associated Press that the fire didn't appear to breach the cabin. The fire was out by 4:18 p.m., the airport said.

Worthington praised crew members who he said did a great job of communicating instructions to passengers.

"They were brilliant," he said. "[The communication] was crystal clear.”

The fire shut down the runway until about midnight local time (3 a.m. ET) but three other runways remained in service.

Passengers were taken to area hotels.

Was asleep as the plane took off. Came to a crashing halt. Smell of smoke. Initially told to stay seated, then shout of evacuate. — Jacob Steinberg (@JacobSteinberg) September 8, 2015

We are heading to a hotel now in a bus. People are shaking the pilot's hand and thanking him. — Jacob Steinberg (@JacobSteinberg) September 9, 2015

The flight-tracking website FlightAware reported delays on departing flights of up to 45 minutes, and said incoming flights were also delayed by the fire.

Boeing is prepared to provide technical assistance to the NTSB following today's incident at McCarran Airport in Las Vegas. — Boeing Airplanes (@BoeingAirplanes) September 9, 2015

Boeing Tuesday offered technical assistance to the NTSB. The plane was fitted with General Electric engines.

In a statement, British Airways said: "Our crew evacuated the aircraft safely and the fire was quickly extinguished by the emergency services at the airport. A small number of customers and our crew have been taken to hospital. All customers have been provided with hotel accommodation, and our colleagues are helping them with anything further they require."

Just landed in Vegas to see this...plane on fire on the Tarmac...people still onboard and running off! pic.twitter.com/ul10hFFw9j — David L. Somers (@DL_Somers) September 8, 2015