The passenger jet was responding to an alert of another aircraft in the vicinity, federal authorities said. A Cold War-era military plane was nearby at the time.

Southwest flight 1496 from Burbank to Las Vegas abruptly descended over 400 feet to avoid a mid-air collision with a Hawker Hunter jet.

The Southwest Boeing 737 departed just before noon local time on Friday for a scheduled flight to Las Vegas. Six minutes after takeoff, a privately owned Hawker Hunter fighter jet crossed less than two miles in front of the Southwest plane, passing within a few hundred feet of the same altitude, according to flight tracking site Flightradar24.

Southwest Airlines said in a statement that two flight attendants are being treated for injuries after the incident.