On September 24, 2015, a DUKW amphibious passenger vehicle (aka “duck-boat”) traveling north on Route 99 in Seattle, Washington crossed the center line into the southbound lanes of oncoming traffic. The vehicle struck a motorcoach that was traveling south, killing five passengers and injuring others in both vehicles. During the on-scene phase of the investigation, National Transportation Safety Board investigators found the front driver’s side wheel had separated from the DUKW vehicle. The subsequent materials laboratory investigation found that the wheel had fractured from the front axle housing due to fatigue cracking. This presentation will highlight the numerous techniques used in this investigation, including non-destructive inspection, electron microscopy, metallography, and finite element modeling, to determine the cause of this accident. This presentation will also discuss the recommendations that resulted from this investigation to prevent future accidents on this unique vehicle.