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

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.

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

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

It is unclear what caused the jolt; United Airlines has said the cause was not severe turbulence, and that it is “working with aviation authorities in the US and Nigeria to understand the cause.”