who can...
..name the object placed secretly while the bandits
were sleeping
I don't think it was a bomb that would have been
final enough
Charlie (River Phoenix) had a long two-by-four shaft of wood slung by a rope on his back as he climbed the structure. At the top of his climb, we see him quietly close a hatch door. The two-by-four was probably an outside fastener for the hatch. This hatch closing probably served to contain the exhaust within the structure, probably carbon dioxide/monoxide. At the same time, Allie (Harrison Ford) was seen quietly sliding a door bolt closed, which probably trapped the intruders inside their compartment.
The fire and explosions resulted from the intruders firing their weapons, which caused leaks or bursts of the ammonia tanks, probably anhydrous ammonia, which is flammable if heated enough. The "ammonium hydroxide" pollution afterwards must have been a result of the burning or mixing with water; I'm not sure how that works.