Complex sentence: The officers drove across the bridge and delivered the flares Unbeknownst to them, the lift span had been raised and as they were making their return trip, their vehicle went over the edge of the open bridge.
Split into simpler sentences: The officers drove across the bridge and delivered the flares. They did not know that the lift span had then been raised, and as they were making their return trip their vehicle went over the edge of the open bridge.