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

After diverting, United Airlines Flight UA613 landed safely back in Nigeria. Four passengers and two flight attendants were taken to hospital with injuries but have been discharged, United Airlines said in a statement. The Federal Airports Authority of Nigeria (FAAN) said an additional 27 passengers and five crew members sustained minor injuries.

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.”

Six were injured on a United Airlines flight from Lagos to Washington after the plane had a technical issue that caused an "unexpected movement."

Six people, four passengers and two crewmembers, were injured on a United Airlines flight Friday from Lagos, Nigeria, to Washington Dulles International Airport in Virginia, according to the airline.

United Airlines Flight 613 traveling from Lagos, Nigeria, to Washington D.C. returned to Lagos after there was “unexpected aircraft movement” early in the morning of Friday, Jan. 24, USA Today Travel and CNN reported.

Several passengers onboard the United Airlines flight UA613 sustained injury as the aircraft made an emergency return to Lagos. The Washington bound flight with 245 passengers, eight flight attendants and three pilots made direct return to the Murtala Muhammed International Airport,