![](https://flightsafety.org/wp-content/uploads/2018/01/humanfactors_jun15_unitedairlines.jpg)
Dutch investigators say they were unable to conclusively identify the cause of a 2018 serious incident in which 23 passengers and cabin crewmembers became ill, causing the pilots of their United Airlines Boeing 767-300ER to return to Amsterdam about an hour after departure on a flight to Newark, New Jersey.