A 19-year-old disc violently broke apart inside the engine of an American Airlines Group Inc. jetliner taking off from Chicago’s O’Hare International Airport on Friday, sparking a fire and touching off a wide-ranging probe into certain General Electric Co. engines, according to investigators and others familiar with the matter.

The accident, which hurled one fragment of the disc onto the roof of a United Parcel Service Inc. facility more than half a mile away, started a fire in a pool of fuel under the Boeing Co. 767’s right wing, National Transportation Safety Board investigator Lorenda Ward said at a briefing over the weekend.

Of the 170 people on board, 21 received medical treatment for injuries sustained in using emergency slides to leave the plane, but all were released from hospitals by Saturday.

The accident was unusual because modern jet turbines are designed to prevent such high-energy parts from being spewed outside the engines. According to one person familiar with the details, the engine-turbine disc had no history of performance problems, failures or mandatory safety fixes.

However, at least four other planes powered by the same GE family of engines have experienced serious engine malfunctions since 2000, including a pair of single-engine failures while airborne.