Just another news story...
http://www.time.com/time/nation/article/0,8599,1138965,00.html

If infact he did yell "bomb" the killing was justifiable. However, if as this article states no one yelled bomb and was just stressed about being in a confined space for X amount of hourse then I don't think it was. Otherwise that mean marshalls could kill anyone with a reaction to any phobia.