(Hillsboro, OR)  --  An investigation continues to determine what caused the release of an unidentified toxic substance that sickened more than two-dozen people in a Hillsboro Shari's restaurant on Saturday.  Investigators say a heating and air conditioning unit on top of the restaurant was likely responsible, but it's still not clear what caused the leak.  Seven people were transported to hospitals, and 19 others were treated on scene.  The victims say they started coughing and experienced a burning sensation in their throats along with dizziness.