At this point, the distinct pops of gunfire being exchanged, are heard from somewhere to the aft of the ship, follwed by three quick thuds. Crystal, from her military experience, recognizes them as frag grenades going off.
No further gunfire is heard after the grenades have gone off.
Down in the Aft Storage, Ethel and Butch see three men in machinist coveralls enter the area, only they aren't machinists.
They are currently donning gasmasks, as they head toward where the pallet is stowed - two with deck pistols out and the leader wearing an armored vest, with a military sidearm, four frag grenades, and a toolkit of some kind.
When they get to the pallet, the leader yells something muffled by his gasmask and he and another man spread out and scan the cargo area in a defensive position (they must have seen their booby trap tampered with).
As this goes on, the third guy gets a pallet-jack and begins to take the bottles of smallpox gas toward the cargo elevator back in the Loading Area...