I'd bet there's a good chance they have ID'd it. All the bullets have been fragmenting. Just because they couldn't get all the fragments, or even the largest fragment, doesn't mean they weren't able to get any fragment large enough to ID.
OTOH, if this is another cell, they could be using different ammunition--as well as a different gun.
The other thing that strikes me about this is that the victim was not stationary. He and his wife were walking to the car. He was moving--though he was probably walking right into the shooter.