AHMEDABAD: The Income Tax Department is investigating a most peculiar case in Ahmedabad. A businessman who voluntarily disclosed unaccounted income of a whopping Rs 13,000 crore is now under the scanner for what seems to be a silly slip - he forgot to pay the tax on the unaccounted income. Mahesh Shah had voluntarily declared Rs 13,000 crore as unaccounted income under the Centre's Income Declaration Scheme (IDS) on September 30, the very last day of the scheme. The IDS offered immunity to those who declared their unaccounted income once they paid the tax.But this is where Mahesh Shah seemed to slip. IT Department officials say Shah failed to pay the first installment of the tax - an amount of Rs 975 crores — by November 30. After this, the Rs 13,000 crores were "declared black".The IT Department then carried out search operations, following which it was revealed that "Shah declared black money on behalf of several big shots of Ahmedabad."Meanwhile, the IT Department is also conducting a search at the offices of Mahesh Shah's accountant and more revelations are expected.