"I'm deeply saddened to hear about Justin Mentell," Shatner wrote on Twitter. "There's no telling how far up the ladder he may have climbed. My sympathies to his family."
Mentell, 27, who played attorney Garrett Wells on the ABC show from 2005 to 2006, was found dead Monday near his crashed Jeep, the Associated Press reports.
His car had gone down a steep embankment near Blanchardville, Wis., and hit two trees.