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.

Six people were taken to hospital, and many more were injured after a United Airlines flight from Lagos heading to Washington D.C. experienced a sudden in-flight movement. The United States has opened a probe into the unexpected movement, as the cause is still unknown.

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

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

The same United Airlines aircraft had a similar issue earlier this week, with a rapid altitude drop prompting a diversion.

A United Airlines flight had to make an emergency landing in Lagos, Nigeria due to a midair incident causing loss of pressure and sharp descents.