Maybe a cop that was overlooked for a promotion, when Moose was hired? Someone leaked the tarot card. Trying to make Moose look bad.
Moose has been doing a good enough of a job of that himself, and it seems unlikely that someone attempting a little job-politics office sabotage would go to the lengths of committing multiple murder.
But the ranges involved are more those for which police marksmen and SWAT snipers train than those for which military sniperrs prepare. And the use of an M16 or other .223 caliber weapon would also be consistant with that possibility.
-archy-/-