A flight to Washington, D.C., returned to Nigeria after “unexpected aircraft movement” caused injuries, United Airlines said. A statement from United emailed to The Hill on Monday said that one of its flights headed for Washington,

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

Six people were seriously injured when a United Airlines flight experienced sharp drops in speed during its journey to Washington Dulles International Airport.

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.