The U.S. Treasury Department issued its first formal warning against potentially abusive investment strategies designed to reduce tax bills and moved Monday to limit one strategy involving exchange-traded funds. The Internal Revenue Service is also scrutinizing prearranged ETF conversions, Section 351 exchanges, in-kind redemptions, box spreads and straddles to determine whether they improperly reduce or defer tax liabilities. The actions follow officials' earlier concerns about the spread of tax-motivated investing techniques and could affect traders, funds and existing or future crypto-related ETF structures. The agencies have not announced a blanket prohibition.