The Nigerian Airspace Management Agency (NAMA) has explained why United Airlines chose to make an air return to Nigeria.

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.

In a statement on Wednesday, Abdullahi Musa, NAMA’s director of public affairs and consumer protection, said the decision to return to Nigeria was made by the pilot based on his assessment of the situation and the perceived safety of landing at the departure airport.

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

In a statement on X, formerly known as Twitter, the NTSB said that it was investigating the January 24 incident involving a United Airlines Boeing 787-8, registered as N27903, which was operating flight UA613 from Lagos Murtala Mohammed Airport (LOS) to Washington Dulles International Airport (IAD).

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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.

Six people were taken to hospital, and many more were injured after a United Airlines flight from Lagos heading to Washington D.C. experienced a sudden in-flight movement. The United States has opened a probe into the unexpected movement, as the cause is still unknown.

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

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