Care to try a really hard murder mystery?

Edward left his car at the top of the hill to go halfway down and argue with the mechanic. Something just wasn’t right about it.
Sure enough, a few minutes later, they saw the car barreling down the hill and plow into a restaurant. There were only two people in there, Edward’s wife, Susan, and an unknown gentleman.
Detective Isis went over the car, but, aside from a little water on the floor board, there was nothing wrong with it. Nevertheless, Isis arrested Edward for a double murder.
How could she be sure?

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