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

A United Airlines flight bound for Washington Dulles International Airport had to turn back to Nigeria Friday after a technical issue with the plane injured multiple people, according to the airline.

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.”

The same United Airlines aircraft had a similar issue earlier this week, with a rapid altitude drop prompting a diversion.

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,

A United Airlines flight from Lagos to Washington Dulles experienced a sudden in-flight movement, injuring six people. The plane made an emergency landing back in Lagos, and the injured were treated and released.