With certain kinds of ammo and depending on the size...SUPPRESSORs...can make a firearm quite quiet.
They’re saying that the guy used a suppressor. Ok. Was it a commercially available one? Or did he make one? Not hard to do at all and can be done using the housing of a Maglite flashlight, a drill press and some solvent cups.
If he did use a commercial one, then this could lead one to think this was planned some time ago. Months ago. Possibly 6 months or more.
First you have the upfront costs to buy the suppressor, fee for the $200 tax stamp, sending the $200 and Form 4, and then waiting for the folks in West Virginia to approve your purchase and enable you to pick it up from your dealer. It takes some time if you do it the legal way.
The last time I got a Tax Stamp for a short barrelled rifle, which falls into the same Class 3/NFA category as suppressors, I waited 8 months.
The original patent was for a “silencer”. But the tacticool crowd loved the word suppressor. They also call some soldiers and sailors “operators”. I always laugh at that term and let the offender know my mom was an operator.