When it comes to arresting drug traffickers and dismantling organized crime, the investigation into a U.S. horse racing operation allegedly laundering money for one of Mexico's most powerful cartels is rare — and difficult to prosecute.

The cases require financial expertise and sometimes years to trace the drug profits back through reams of bank, tax and property records. But the money cases are the ones that can dismantle a syndicate by choking off its cash supply.

In this latest case, federal agents raided an Oklahoma ranch, a New Mexico quarter horse race track and sites in Texas on Tuesday, where prosecutors allege a brother of a leader in the Zetas drug cartel was using a horse-breeding operation to launder money. Eight people have been arrested.