Silencers alone are not that quiet like in the movies. You still need ear pro if you're shooting a suppressed gun because of the sonic boom of the round as it leaves the barrel.
Granted, in the crime area we are talking about, you would not be able to tell direction but the shots would be audible to the hikers. It is also possible that it would not be recognized as gunfire but someone would surely go "what was that?", enough to generate comments to law enforcement afterwards.
To get to that level of quietness the killer would need here and to not alert anyone around in that wooded area (
my assumption is that the crime scene is about a mile away from the closest hiker/person) one would need a can, subsonic rounds (.45 acp is naturally subsonic), and quick action. At that point your slide would literally make more noise than the round. Possible but...
You would also need to get through a complete ATF background check and wait an average of 190 days before you can even get your suppressor (
ATF Wait Times). And I won't even go into the costs of threaded barrels.
So to get to that level of planning (and knowledge) to be able to have that in your possession to commit this crime means a LOT of premeditation. Meaning that this person had kept such a quiet profile thus far and had such a clean background to be able to get a can and go kill two girls on a hiking trail. Again, not impossible but it starts to become less and less likely.
NOTE: Still waiting on verification, so treat the above as MOO.