Complex sentence: Pursuing a lead which led him to an abandoned warehouse, he discovered thousands of barrels containing bags of a yellow powder labeled to be radioactive and which was later confirmed by the IAEA that this powder was Yellowcake uranium.
Split into simpler sentences: Pursuing a lead which led him to an abandoned warehouse, he discovered thousands of barrels containing bags of a yellow powder labeled to be radioactive. It was later confirmed by the IAEA that this powder was Yellowcake Uranium.