|I haven't seen the earlier chapters, but even looking just at this one, I can see that the quality of writing is very high and this story has great promise.
The two things that I would suggest to work on is perspective and pacing.
On perspective or voice, the narration is written from the viewpoint of first one character then the next. This allows the reader to get a balanced view of events, but it can blur the focus and impact of the writing.
On pacing, here's a quote that demonstrates this:
""Just as suddenly as it had started the siren cut out, the audible clicks as the deadbolts drew back unlocking the doors cut through the tension.
Now the dingy parking structure was ghostly quiet and amazingly seemed vastly empty. Devoid of life except the huddled bodies of the squad.""
This presents the events in a kind of abstract continuous present, when they are worthwhile narrating as if they were a series of surprises impacting on the reader one by one. It also splits two closely related sentences.
I'm not able to do this justice, but here's an attempt to show these events in a kind of visceral present:
"The oppressive wail of the siren stopped suddenly, leaving a cold vacuum. Shawn became aware of the sound of his own ragged breathing and the dull ache in his neck from bending over in the van.
A series of loud clicks sounded near his head made him twist suddenly before he realised it was just the deadbolts withdrawing on the door.
He looked at Andy who just nodded in response. Another member of the squad coughed dryly on the dust that had been shaken up. Shawn grasped the heavy handles and the door's weight gave smoothly, allowing a waft of cool air in.
The dingy parkhouse was eerily silent now except for muted sounds, presumably the echoes of the squad's movements.""
One other point, more to do with description than anything else. Shawn and Andy seem to have real possibilities as characters, but beware of cliches such as "gentle giant". These are your characters so reach deep and find a way to tell what's unique and memorable about them.