The Internal Revenue Service allowed undocumented workers to collect $4.2 billion in refundable tax credits last year, a new audit says, almost quadruple the sum five years ago. Although undocumented workers are not eligible for federal benefits, the report released Thursday by the Treasury Inspector General for Tax Administration concludes that federal law is ambiguous on whether these workers qualify for a tax break based on earned income called the additional child tax credit. Taxpayers can claim this credit to reduce what they owe in taxes, often getting refunds from the government. The vagueness of federal law may have contributed...