A brawl between two passengers that broke out Monday forced a Southwest Airlines plane bound for San Francisco to turn around mid-flight.

The captain declared an emergency to receive priority handling from air traffic controllers to land the Boeing 737-300 back at Los Angeles International Airport, ABC's Channel 7 reports.

The pilot on board Southwest Flight 2010 taxied to a gate where police entered the cabin.

"Our initial information is that flight attendants swiftly coordinated with pilots on the flight deck to get the aircraft on the ground safely and quickly after a physical altercation by one passenger against another," said Melissa Ford, a spokeswoman for the airline.

It’s unclear what sparked the altercation, but there are reports that it could have been over legroom

Ford didn't provide details but described it as a "rapidly escalating situation" between two passengers who weren't traveling together.

Passengers expressed their frustration with the delay on social media.

The FBI is now handling this investigation and say they're still questioning a passenger.