(Mattawa, WA)  --  Six people are injured, two critically, following an explosion at the Priest Rapids Dam in Grant County, Washington.  Grant Public Utility District officials say the blast happened around 4:15 yesterday afternoon at the dam, located along the Columbia River.  Authorities say it may be several days before the cause of the explosion is determined.  Utility spokesman Thomas Stredwick told "The Oregonian" that there's no evidence of foul play or an act of terrorism.