FORT WORTH, Texas — Federal safety officials are looking into a flight that American Airlines operated to Hawaii using a plane that lacked extra emergency equipment required for long over-water trips.

Federal Aviation Administration rules require such planes to carry extra oxygen for medical emergencies and an extra canister of fire extinguisher. That’s because the planes fly far from airports where they could make an emergency landing.

An American Airlines flight from Los Angeles was on its way to Honolulu on Aug. 31 when an employee on the ground realized that the airline had sent the wrong plane, spokesman Casey Norton said Monday. He said the airline completed the flight but cancelled the return trip, which would have used the same jet, and flew the plane flew back to Los Angeles without passengers.

Norton said American notified the FAA of the mistake and changed its software to make it easier for workers to match the correct plane with the route. An FAA spokesman said the agency was investigating.

FAA spokesman Les Dorr said that to the agency’ knowledge there has never been an incident quite like this one.

The mistake occurred about two weeks after American switched from Boeing 757s to Airbus A321s on the Hawaii route. Some of its A321s are certified for over-ocean flights but some are not. Norton said both types of A321 have the same engines and the same number of life rafts and flotation devices.