Complex sentence: The catastrophe, while officially classified as an accident, has led to several conspiracy theories that persist to this day, and often propose that the crash was an assassination, which has variously been blamed on the Soviets, British or even a dissenting Polish faction.
Split into simpler sentences: The catastrophe, while officially classified as an accident, has led to several conspiracy theories that persist to this day. Said theories usually assume that the crash was a result of an assassination attempt, which has variously been blamed on the Soviets, British or even a dissenting Polish faction.