(Milwaukie, OR)  --  The suspect in a fatal hit and run in Milwaukie is back behind bars arrested on a new witness tampering charge.  Police say 30-year-old Kevin Derrick is charged with running down 50-year-old Robert Keys of Gladstone in September, killing him.  Derrick and his girlfriend, Alysha Esser, are accused of assaulting a witness to keep them from testifying.  Derrick is being held on a million dollars bond.  Esser remains at large.