The U.S. National Transportation Safety Board , NTSB, announced on Monday that it is investigating an incident involving a United Airlines flight on Friday from Lagos, Nigeria, to Washington D.C.

The investigation follows reports of sudden aircraft movement that resulted in injuries to both passengers and cabin crew.

The NTSB stated that the incident occurred aboard United Flight 613, a Boeing 787, while it was flying through Ivory Coast airspace.

Aviation authorities in Ivory Coast have handed over the investigation to the NTSB, which is expected to release a preliminary report within 30 days.

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United Airlines confirmed that the flight, carrying 245 passengers, three pilots, and eight flight attendants, was forced to return to Lagos after the unexpected movement of the aircraft, which was accompanied by a technical issue.

The plane safely landed back in Lagos, and four passengers and two flight attendants were taken to a hospital for treatment of minor injuries. All have since been released.

“We are working with aviation authorities in the U.S. and Nigeria to understand the cause,” United Airlines said in a statement.

Reported by Reuters