1. Guns are traceable for the most part and can be used specifically in signature forensics a lot easier than a knife. So, if LE ever found the gun that was used: you’ll find the killer as well. Also, silencers aren’t like in the movies: they’re still rather loud and don’t muffle all the noise.
2. The act of killing with a gun is way less gruesome and way less personal than overpowering someone and plunging a knife in
their chest and watching the life leave their eyes through exsanguination.
3. Because it’s literally just as easy to obtain a gun as it is a knife, this must speak to the psychological profile of someone who decides to kill 4 other people in that manner: It’s obviously not just about wanting to kill— (that can be managed several other ways that are less personal and less messy; i.e. Guns, a house fire, etc…) it’s the manner in which this person did it and that’s also probably why the FBI and the Behavioral Units have been called because of the brutality of the crimes and the nature of the murders.
4. I’m assuming the Medical Examiner will also be able to ascertain if there was more than one killer by the signature of the knife wounds— if all of them are mostly consistent: it’s probably just one killer. I think it’s also just one perpetrator because it takes a special kind of killer to murder this way and finding a buddy to do it with you creates many more possibilities of problems.
5. I agree that it was probably random and IMO, I think it’s a Thrill Killer which is a serial killer subtype also as evidenced by the FBI on scene. These types of killers can obtain sexual gratification just by the act of killing, so there still could be a sexual component to the crimes, just not in the normal sphere of most killers.