In a statement on X, formerly known as Twitter, the NTSB said that it was investigating the January 24 incident involving a United Airlines Boeing 787-8, registered as N27903, which was operating flight UA613 from Lagos Murtala Mohammed Airport (LOS) to Washington Dulles International Airport (IAD).

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.”

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

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,

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.