What is the government's burden of proving that non-taxable income does not derive from those sources?

The government also sufficiently investigated all reasonable leads to non-taxable sources. The government meets its burden when "it investigates reasonably possible sources of non-taxable income and explores whatever leads the taxpayers or others may proffer," United States v. Mastropieri, 685 F.2d 776, 785 (2d Cir.), cert. denied, 459 U.S. 945, 103 S.Ct. 260, 74 L.Ed.2d 203 (1982), and thus, by showing that non-taxable income did not derive from those sources, "negat[es] ... [all] reasonable explanations by the taxpayer inconsistent with guilt." Holland, 348 U.S. at 135, 75 S.Ct. at 135.