The Bengal government has challenged a Calcutta High Court order for a CBI probe into post-poll violence in the Supreme Court, saying the agency will not conduct a fair investigation. "The Central Bureau of Investigation is working at behest of the Centre, don't expect a fair investigation," said the government of Chief Minister Mamata Banerjee, which has been at loggerheads with the agency for several years. The CBI, Bengal said, is busy registering cases against office-bearers of the Trinamool Congress. Mamata had dubbed the agency a tool of the Modi government, used to punish political opponents. Earlier, the Calcutta High Court had ordered a CBI probe into "heinous crimes of murder and rape" in the state after the assembly elections, in which Mamata's Trinamool Congress won a landslide victory in face of the BJP's mammoth election machine.