Six people were seriously injured when a United Airlines flight experienced sharp drops in speed during its journey to Washington Dulles International Airport.

A United Airlines flight from Nigeria to the US was forced to make an emergency landing in Lagos early Friday. The flight from Lagos to Washington D.C. turned back after the aircraft — a Boeing 787-8 — experienced a technical issue and "an unexpected aircraft movement,

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.”