A United Airlines flight bound for Washington Dulles International Airport from Lagos, Nigeria, was forced to make an emergency return mid-flight early Friday after a technical issue with the aircraft that put six people in the hospital, an airline spokesperson said. 

The plane experienced an “unexpected aircraft movement” during flight, and aviation authorities in the United States and Nigeria are investigating the cause, the spokesperson said. 

Four passengers and two crew members aboard flight 613 were seriously injured, Nigeria’s Federal Airports Authority said in a statement, and 27 passengers and five crew members suffered minor injuries. There were 245 passengers and 11 crew members on board, it said. 

The passengers who were hospitalised after landing have since been discharged, the United Airlines spokesperson said. 

Tracking data from Flightradar24 shows the aircraft’s speed drop sharply twice in quick succession about 75 minutes into the journey while flying at an altitude of about 36,000 feet. 

It slowed from over 500 knots to 40 knots, resumed its original speed, and then slowed to 70 knots before resuming again. 

A third, lesser drop in speed occurred about seven minutes later. Shortly afterward the plane reversed course back to Lagos, over land above Ivory Coast, according to the tracking data. 

The flight took off from Murtala Muhammed International Airport about midnight local time, and a pilot communicated distress about 1.20am, the aviation authority said. 

The plane landed safely back in Lagos about 3.22am, it said. 

The plane forced to carry out the emergency return on Friday was a Boeing 787-8, and the event was the latest in a number of high-profile incidents involving planes from the embattled manufacturer. Boeing did not immediately respond to a request for comment late Friday. 

-Frances Vinall, Washington Post