NEW YORK — A bus driver accused of being so tired that he caused a crash that killed 15 passengers was found not guilty on all felony charges by a jury on Friday, underscoring the difficulty in prosecuting so-called drowsy driving cases.

The driver, Ophadell Williams, was returning to New York from an overnight trip to the Mohegan Sun Casino in Connecticut in March 2011 when his bus struck a guardrail on Interstate 95, flipped over, and smashed into a signpost with enough force to shear off part of the roof.