Complex sentence: The Dutch parapsychologist Wilhelm Tenhaeff has written Croiset had genuine psychic powers due to the information he had given in police cases, however it was discovered that much of Tenhaeff's data was fraudulent.
Split into simpler sentences: The Dutch parapsychologist Wilhelm Tenhaeff has written Croiset had genuine psychic powers due to the information he had given in police cases. However it was discovered that much of Tenhaeff's data was fraudulent.