LIVE OAK, Fla. (AP) - A woman's 53-year-old son was charged with her murder after investigators discovered her remains in a field, Suwannee County Sheriff Tony Cameron said.  Authorities believe William Robert Elliott ran over his mother's remains with a piece of farm equipment after he burned her body, Cameron said. The remains of 79-year-old Martha Elizabeth Elliott were found Sunday, ending a search that began Wednesday.  William Robert Elliott was being held in the Suwannee County jail Monday evening on charges of first-degree murder and abuse of a dead human, according to a jail official. Bond has not been set.  Martha Elliott was last seen Nov. 6, Cameron said, and investigators aren't sure why she was killed. Investigators think William Robert Elliott acted alone, Cameron said.