I have not seen any of the silencers involved in this case... That said they sound rather crude.
First, suppressor is more accurate. What you hear in a movie is nothing like what a suppressed gunshot really sounds like. Its still loud, and it still sounds like a gunshot, its just not going to rattle the glasses in the cupboard anymore. Thats why I say the term "suppressor" is much more accurate. Silencer implies that you won't hear a gunshot, and that completely false. These murders were most certainly heard, just not half a mile away.
The purpose of a silencer is to slow down the bullet and disrupt the flow of the gasses as they exit the firearm. Half of what you hear is the bullet actually breaking the sound barrier. If you do a quick video search for "See Through Silencer" there are videos where you can see the insides and they actually show shots in slow motion. Really cool IMO... These suppressors don't use a filler material, they have baffles cut with a CNC machine to disrupt the gasses. Same concept, BUT THE BULLET DOESN'T TOUCH THE INSIDE OF THE SUPPRESSOR with the newer CNC machined designs. The suppressors used in this crime sound more like what I could make in my garage...
EDIT:
See Through Suppressor in Super Slow Motion (110000 fps) - YouTube
Thread an extension onto the barrel, and drill holes through the extension to vent gasses. Pack washers and steel wool over the extension, and seal it. The steel wool filler or fluff exits the barrel with the bullet because that stuff gets everywhere... Its a forensic goldmine really.