A U.S.-led investigation into a series of engine failures on Airbus's smallest jet, the A220, is studying whether a software change allowed unexpected vibrations that tore parts and forced three emergency landings, several people familiar with the case said.

The airline Swiss halted its fleet of A220 jets for more than a day on Oct. 15 after a third flight in as many months was forced to divert or return to base with engine damage. Engine maker Pratt & Whitney has urged checks on similar engines worldwide.

Investigators are focusing their attention on a recently revised version of engine software that may have allowed parts inside the engine to be set in a way that caused mechanical resonance or destructive vibrations, two of the people said.