AKRON, Ohio (AP) -- The effort to nail down the source of the massive blackout turned Saturday to northern Ohio, where the power system is operated by FirstEnergy Corp., the nation's fourth-largest investor owned utility.Akron-based FirstEnergy has 16 power plants and an annual revenue of more than $12 billion, with a service area that stretches from Ohio to New Jersey.The company is accustomed to scrutiny.Its Davis-Besse nuclear plant east of Toledo has been shut down since February 2002, when it was closed for maintenance. A month later, a leak was found that had allowed boric acid to eat nearly through...