There were 30 odd witnesses to an explosion. The single thing that makes it a possibility though (for me) is that vertical stabilizers simply don't separate from airplanes in flight. It would be like your nose separating from your face in a stiff breeze.
I guess you've never seen the video of the F-117 losing one of its vertical stabilizers during a fly-by. It was pretty dramatic. As was the fireball when it hit the ground. I'm guessing it must have been a shoebomber.
Actually it does happen sometimes:
http://www.ntsb.gov/pressrel/2000/000210a%5F.htm
http://www.ntsb.gov/scripts/search/ch_smpl.idq?CiCatalog=d%3A%5CInetPub%5C&HTMLQueryForm=%2Fsearch%2Fsrch_ch.htm&CiBookMark=N-2d547d0-274c-b&CiColumns=filename%2Csize%2Crank%2Ccharacterization%2Cvpath%2CDocTitle%2Cwrite&CiFlags=DEEP&CiRestriction=%28tail+seperation%29AND+NOT+%23vpath+*%5Citsa*+AND+NOT+%23vpath+%5Ccustomer*+AND+NOT+%23vpath+*%5Cspecial*+AND+NOT+%23vpath+*%5C_vti_*+AND+NOT+%23vpath+*%5Cexchange*+AND+NOT+*%5C*.log+AND+NOT+*%5C*.asp*+AND+NOT+*%5C*.ascx+AND+NOT+*%5C*config*+AND+NOT+*%5C*.vb*+AND+NOT+*%5C*.xml+AND+NOT+*%5C*.wmz+AND+NOT+*%5C*.emz+AND+NOT+*%5C*.mso+AND+NOT+*%5C*.jar+AND+NOT+*%5C*.asa+AND+NOT+*%5C*_HIDDEN.*+AND+NOT+*%5C*.png+AND+NOT+*%5C*.mno+AND+NOT+*%5C*.js+AND+NOT+*%5C*.dwt+AND+NOT+*%5C*.bak+AND+NOT+*%5C*.css+AND+NOT+*%5C*.cfm&TextRestriction=tail+seperation&CiMaxRecordsInResultSet=300&CiMaxRecordsPerPage=10&CiScope=%2F%2CCIS2%2CCIS3%2CCIS4%2CCIS5%2CCIS6&CiTemplateChoice=1&SortProperty=rank&CiSort=rank%5Bd%5D&CiForceUseCi=TRUE&CiBookmarkSkipCount=10
Not very often, but they're parts just like everything else and sometimes parts fall off.
