Complex sentence: Retz is rumored to have also produced other animated short stories on his own time, This includes an animated documentary about the mass arrest of more than 13,000 Jews in the Vel'd'Hiv Roundup in Paris that this is thought to have been destroyed by the authorities.
Split into simpler sentences: Retz is rumored to have also produced other animated short stories on his own time. These include an animated documentary about the mass arrest of more than 13,000 Jews in the Vel'd'Hiv Roundup in Paris that this is thought to have been destroyed by the authorities.