Complex sentence: The captain later ordered his crew to turn off the ventilation system as it was feeding air to the fire, and an unintended result was that smoke was able to enter cabins via the door vents and suffocate trapped passengers.
Split into simpler sentences: The captain later ordered his crew to turn off the ventilation system as it was feeding air to the fire. Unfortunately, smoke was then able to enter cabins via the door vents and suffocate trapped passengers.