United Airlines said it was cooperating with U.S. and Nigerian investigators to determine what exactly happened on the flight.

A United Airlines flight from Africa to Washington D.C. had to turn around on Thursday after a "technical issue" caused "unexpected aircraft movement," the airline said.

The authorities in Nigeria said that the United Airlines plane had to return to Lagos, Nigeria, after the episode. Six people were treated at a hospital, officials said.

Flight data from Flightradar24 shows that the plane abruptly dropped speed twice during the flight.