The first is the older type:
The second is the newer type
You can see that both have similar wounding potential, while the newer type, being heavier, retains it's velocity and accurracy out to longer ranges, and also provides somewhat better penetration of intervening materials.
Indeed, if it was a certainty that a 5,56mm weapon had been used in all the cases; I understand that's not yet at all certain. And I might agree if the first five shootings had resulted in a ratio of about 1 victim killed out of every 3 hit, the usual ratio for 55-grain M193 ball at distances under 150 meters, as experienced by US combat troops in Vietnam at such ranges. Reports from the recent efforts in Afghanistan indicate that the M855 ball ammunition, particularly when used in the shorter-barrelled M4 carbine, is even less effective in terms of stopping effect, if not accuracy. Accordingly, I'd bet that those first several shots were using softpoint or hollowpoint bullets, which though conjecture makes for a pretty fair working assumption.
Pieces of recovered bullet jacket material can be identified and linked to others from the same lot, fired or unfired, by neutron analysis examination, as well as other mettalurgical testing, whether enough of the deformed bullet is available for comparison testing or not. That'll take a little longer than just sticking a recovered bullet and an exemplar under a comparison mike, but happily, the FBI lab is quite nearby, and the recovered evidence will have high priority for processing, I'm sure.
-archy-/-