LAS VEGAS — Federal safety investigators are trying to determine what caused a British airliner’s left engine to burst into flames as it hurtled down a Las Vegas runway, forcing the pilot to abort the takeoff and passengers to flee the burning plane.

Passengers on the London-bound Flight 2276 reported hearing two loud booms Tuesday before flames and the smell of burning rubber sent them dashing to the inflatable evacuation slides, unsure if they were fleeing a bomb or a blown tire.

“Everyone was screaming, ‘Just keep on running,”’ said Karen Bravo, a 60-year-old who abandoned the flip-flops she had taken off minutes before to settle in for the 10-hour flight to Gatwick Airport south of London.

When the running stopped and they turned around, there was nothing but black smoke and flames.