A United Airlines flight was traveling from Lagos to Washington, D.C. when it experienced "sudden aircraft movement."

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 United Airlines flight from Africa to Washington D.C. had to turn around on Thursday after a "technical issue" caused "unexpected aircraft movement," the airline said.

Six people were taken to hospital with injuries but have been discharged, United Airlines said.

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.

Nigerian Airspace Management Agency, NAMA, has said contrary to reports, United Airlines flight UAL 613, which departed Lagos en route Washington with 256 passengers and 11 crew members on January 24,

Six people were injured on a flight from Nigeria to Virginia. The airline also saw a series technical issues in March.

The plane headed back to Nigeria after experiencing "unexpected movement" mid-flight on Friday, Jan. 24, according to multiple reports

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.”

In an update, Nigerian officials said an additional 27 passengers and 5 crew members suffered minor injuries in the incident. In all, 38 were hurt.

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