The National Transportation Safety Board is investigating after a United Airlines flight experienced "sudden aircraft movement," leaving dozens of passengers and crew injured, including six people who were hospitalized.

United Airlines flight 613 was flying from Lagos, Nigeria to Washington Dulles International Airport on Friday when the incident happened, the NTSB said on social media. The plane, a Boeing 787-800, was flying in the Côte d'Ivoire's airspace when it experienced what United and the NTSB have both called "sudden aircraft movement." The flight then returned to Lagos, landing at around 3:20 a.m. local time.

At Lagos' Murtala Muhammed International Airport, the plane was met by medical responders, firefighting services and aviation security, the Federal Airports Authority of Nigeria said in a news release. The agency said responders were on standby just four minutes after the flight's pilot reported distress. The responders were on site before the plane landed, according to FAAN.

Of the 245 passengers on board, 31 had injuries. Four passengers had "sustained serious injuries," Nigerian officials said, and were transported to an area hospital. Another 27 passengers had minor injuries and were treated at clinics. There were also eight flight attendants and three pilots. Two crew members were hospitalized, and another five reported minor injuries, but FAAN did not specify which roles those crew members had.

The aircraft itself was not damaged in the incident, FAAN said.

The NTSB said that a preliminary report of its investigation will be available online in about 30 days.

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