(Wilsonville, OR)  --  An investigation is ongoing at Coffee Creek Correctional Facility after an envelope filled with a white, powdery substance forced evacuations yesterday.  Tualatin Valley Fire and Rescue responded to the prison yesterday morning after the envelope was discovered in the mail room.  Authorities say four mail room workers were exposed to the unknown substance.  The four workers were evaluated at a hospital as a precaution, but authorities say they weren't experiencing any symptoms.  It's not yet clear what the substance was, but a HazMat crew has decontaminated the facility.