Fire investigators are examining evidence to determine if a Thursday morning fire at an industrial site near Mitchell International Airport was a case of arson.  Explosions witnesses heard were likely tires blowing up on the four vehicles which were burning.  Firefighters had to use foam instead of water on the burning dump trucks.  The fire filled the air on Milwaukee’s south side with thick, black smoke near the Plainfield Curve of the Mitchell Interchange.  No injuries were reported.