CHICAGO (AP) — Pilots were forced to abort a takeoff and evacuate passengers from a burning American Airlines flight on a runway at Chicago O’Hare International Airport.

A federal official says the airliner experienced a rare and serious type of engine failure Friday. It’s called an “uncontained engine failure” and it happens when engine parts break off and are spewed outside the engine.

The official spoke to The Associated Press on condition of anonymity because the person was not authorized to speak publicly about the incident.

Flames and heavy black smoke poured from the side of Miami-bound American Airlines Flight 383 as it sat on the runway after the aborted takeoff. Officials say the incident left 21 people injured.

The National Transportation Safety Board will conduct an investigation into the incident.