On a twisty, snow-lined hill that the locals call "Devil’s Backbone," a 12-ton semi-trailer came loose from its tractor and plowed into an oncoming line of pre-dawn commuters. At 40 miles per hour, the trailer struck the side of one pickup truck and careened head-on into another, killing the drivers of both vehicles. At first the Jan. 24, 2014, crash on U.S. 50 in Cincinnati’s eastern suburbs drew only the attention of Ohio authorities, who faulted the semi driver for not properly inspecting the hitch that holds the trailer to the tractor. But 17 months later, the National Highway Traffic Safety Administration has raised the possibility that the hitch was defective.