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.