A lumber mill in Maine exploded and burst into flames Friday – as authorities declared the terrifying situation a “mass casualty event.”  At least five people have been injured – with one in critical condition – after an inferno erupted at Robbins Lumber mill in Searsmont, about 90 miles from Portland, at around 11 a.m.  “We have dumped all of the resources from the whole county over to that area,” Waldo County 911 director Mike Larrivee said Friday afternoon, according to local news outlet WMTW .  Officials declared the incident a “mass casualty event” not because there were any deaths, but because there are believed to be more victims than local authorities are equipped to handle, the outlet reported.  Horrifying photos and video footage shows giant, thick plumes of smoke billowing from the ablaze mill.…