One person was killed and three others seriously injured Tuesday when a motorist crossed the center line and struck two oncoming cars head-on just across the county line in DeKalb County.The fatally injured man, Edgar “Louis” Madewell, 72, of Warren County, was ejected from his 2007 Ford Expedition. Investigators say he was not wearing a seatbelt and that wearing one may have saved his life.The accident happened Tuesday at 12:20 p.m. on Highway 56 (Smithville Highway).Investigators say Joshua Johnson of Smithville crossed the center line as he was negotiating a curve, heading northbound (away from McMinnville). Johnson’s 1997 Oldsmobile slammed head-on into a southbound 2008 Mazda driven by Virginia Hendrixson of Liberty.