Sun Prairie police say the Tuesday evening explosion in the city’s downtown area killed one firefighter, critically injured another and destroyed several buildings.  The first explosion was reported shortly after 7:00 P-M.  The most recent update lists 15 people injured, including the dead firefighter and a second firefighter in critical condition.  A search-and-rescue operation turned up no additional victims in the damaged buildings.  The problem started when a contractor apparently punctured a four-inch natural gas main, causing a major leak.  More than 100 people were displaced in the evacuation which followed the explosions and fires.