Four people were injured in an explosion in Shelton Tuesday afternoon. According to Central Mason Fire & EMS, crews were dispatched to an explosion with multiple injuries in the 500 block of Grandview Avenue about 12:40 PM. Upon arrival, emergency responders were told that the four victims had “self transported” to Mason General Hospital. Three of the injured are in their 20s while the fourth is a teenage male. Two of the victims have been transferred to Haborview Medical Center in Seattle with burn injuries. The cause of the explosion is being investigated but authorities suspect it was from the manufacture of a controlled substance as the internal walls of the house sustained explosive damage. Shelton Police Department is leading the investigation. The Washington State Patrol also assisted with this incident.