With each passing day, the mystery grows deeper surrounding the Pilatus PC-12 crash last week in Butte, Montana. All 14 Californians on board died, including seven small children. They were on their way to a ski vacation at Yellowstone Club, near Bozeman, Montana. The single pilot-65-year-old Air Force veteran Bud Summerfield-requested a diversion to Butte from the filed destination, Bozeman. Why he decided to divert is among the most profound questions facing the NTSB. The agency is considering several possible reasons for the diversion, including low fuel state, mechanical problems (though the pilot reported none to ATC), a medical emergency on board, a phone call requesting a pick-up in Butte, and others. The NTSB is also examining weather, focusing on the possibility that the big Swiss turboprop single might have flown through an area of heavy icing conditions. Despite the excessive number of passengers, overloading is now considered less likely, since many of the children were infants or toddlers. Investigators are still considering center of gravity issues. For more insight into the breadth of possible answers in this tragic crash, click here for our editor's web exclusive.