There are once again four Dreamliners on the Boeing South Carolina flight line.

A Boeing spokeswoman would not say what happened Thursday, but there are two likely explanations.

Either the North Charleston planemaking plant has rolled out its third locally assembled 787, or the second S.C.-built jet, which sat parked outside for weeks before experiencing a fiery engine failure late last month, has been equipped with a new General Electric engine and returned to the flight line.

Last week, the National Transportation Safety Board, which is leading the investigation into the July 28 pre-flight taxi test mishap, provided an update on the engine failure. According to the agency, one of the GEnx-1B engine’s central drive shafts “fractured” toward the front of the engine between its booster and compressor.

That led the shaft to slide backward and caused blades in the engine’s low-pressure turbine to collide with stator vanes in that turbine. Pieces of those parts then were ejected out the back of the engine, causing a small fire, briefly shutting down the airport and generating international headlines.

The NTSB has provided no further update as to why that shaft broke.

On Wednesday evening, one of the other 787s on the flight line — one of the two built in Everett, Wash., and flown to North Charleston a month and a half ago — took a test flight to Myrtle Beach, according to the flight-tracking website Flight Aware.

All four planes on the flight line, which runs parallel to South Aviation Avenue, are eventually to be delivered to Air India.

The first S.C.-built 787 was slated for delivery in June, but the Indian government, which owns Air India, had not approved Boeing’s compensation package for more than three years of delivery delays. The government has since signed off on the deliveries, but no schedule for the hand-overs has been announced.