From the manuafactuer’s webpage for that model:
“This model in 9mm wears a 3-in. barrel. It finds its operational success as a magazine-fed, non-auto-ejecting single-shot functioning in a rotational bolt action.”
This is a feature, not a bug. Casings make noise when they eject and hit the ground. If you want a truly silent gun, you have to manually expend the casing. Preferably into your hand to leave no trace.
Which, also means… the gunman intentionally left the casings there.
To paraphrase Jay-Z…
I’m not a gun owner but I know a little bit, enough that I’m going to correct you when your reporting is shit.