Complex sentence: The increasing pressure finally burst the inner cabin wall; now fed by nitrogen-buffered ambient air, the fire decreased in intensity and eventually put itself out, but produced large amounts of smoke, which killed the astronauts.
Split into simpler sentences: The increasing pressure finally burst the inner cabin wall; now fed by nitrogen-buffered ambient air, the fire decreased in intensity and eventually put itself out. However, by the time the fire was put out, it had already produced large amounts of smoke, which killed the astronauts.