The correct choice in this context is "but."
The revised sentence would read:
"And fatalities rose in 2012, snapping a six-year streak of reduced deaths. In reporting the increase, the National Highway Traffic Safety Administration stated that ‘it is too soon to speculate on the contributing factors,’ but the potential role of distracted driving can’t be ignored."
Using "but" effectively contrasts the statement about uncertainty with the idea that something (distracted driving) should still be considered.