Envisioning they shoot for 5-8 minutes and leave by the staircase with nothing in their hands and enter another room booked by a friend that’s a few floors up or down. Easey-Peasey.
They’re long gone prior to any response can be assembled. Dodging or blocking security cameras is another story.
There would be a trail of GSR following their path of retreat if they actually exist and were perpertrators. Rugs, doir handles, bannisters would contain some GSR given the amount of rounds fired and the manner in which tgey were fired.