Nearly 40 people were injured, with six seriously injured, when a Boeing aircraft operated by United Airlines suddenly plunged in mid-air, forcing the fight to make an emergency landing.

The US National Transportation Safety Board is investigating a United Airlines Holdings Inc. flight that turned back to Nigeria after a “sudden aircraft movement” occurred, injuring passengers and crew.

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.

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,