The U.S. National Transportation Safety Board said on Monday it is investigating a United Airlines flight on Friday from Lagos, Nigeria to Washington D.C., in which a sudden aircraft movement led to injuries of passengers and cabin crew.

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.

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