Twenty-seven people associated with British Airways Flight 2276 were treated at the Nevada city's Sunrise Hospital and Medical Center, spokeswoman Sylvia Song said. All had been released by Wednesday afternoon.

Most of the injuries came as passengers slid down the four inflatable chutes while evacuating the Boeing 777, Clark County Fire Department Deputy Chief Jon Klassen said.

The incident happened shortly after 4 p.m. (7 p.m. ET) Tuesday,when the aircraft's left engine caught fire at McCarran International Airport, according to the Federal Aviation Administration.

Exactly why that happened was one of authorities' looming questions.

Another was why the fire continued until firefighters -- who witnesses said arrived within minutes of the fire and smoke erupting -- doused it.

Thick black smoke billows out of British Airways Flight 2276 on Tuesday in Las Vegas.

The British Airways jet's fire indication light did come on at some point, the source close to the investigation said. And the plane did have fire suppression equipment that was deployed, but it didn't extinguish the fire.

Investigators are looking into a few possibilities such as whether or not the fire suppression equipment worked properly or whether a fuel line ruptured, causing the fire to spread.

A thud, then an evacuation order

Jay Jennings was one of the 159 passengers, along with 13 crew members, on the London-bound jet when he heard a big thud as the flight was about to takeoff. He lifted his window shade and "just saw flames on the engine," he said.

For a few frenetic minutes, frightening plumes of black smoke engulfed the London-bound jet.

The plane stopped and sat for what felt like a minute, Jennings added. Then the captain came on the intercom and told the passengers there was an emergency and they needed to evacuate.

When one of the emergency doors opened, smoke poured in. "Not safe, not safe," someone said, Jennings recalled.

Paul Berberian, who had just landed on a flight from Denver, said that he didn't know anyone was on the plane until "the slides popped up."

"Five seconds later people were just flying down ... and running away," said Berberian, who estimated firetrucks were on the scene within two minutes.

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CNN's Dan Simon, Ed Payne, Steve Almasy and Artemis Moshtaghian contributed to this report.