Police in Los Angeles have arrested nine people and seized around $70m (£43m) in cash during mass raids on a series of businesses.
Officers said they suspected the money was being laundered on behalf of Mexican drug cartels.
In the dawn operation, dubbed Fashion Police, about 1,000 officers searched dozens of businesses in the city’s Fashion District.
Police found $35m in cash in one warehouse alone.
Assistant US Attorney Robert E. Dugdale said that Los Angeles had “become the epicentre of narco-dollar money laundering with couriers regularly bringing duffel bags and suitcases full of cash to many businesses”.
“The Fashion District in particular seems to be where this is thriving at the moment,” he explained.
But LA Fashion District Executive Director Kent Smith said most businesses in the area were legitimate.
“All of us are very surprised by this,” he told AP.
Agents searched around 50 businesses and found large amounts of cash.
Approximately $90m were seized in total, of which some $70m were in cash, a spokesman for the attorney’s office said.
Prosecutors alleged that one business had laundered $140,000 paid in ransom payments to secure the release of a US citizen held hostage by the Sinaloa drug cartel in Mexico.
Officials said Mexican drug traffickers were using US businesses as middlemen to get their illegally obtained US dollars converted into Mexican pesos.
In what is known as Black Market Peso Exchange, they use their illegal US dollars to buy goods in the US and export them to Mexico, where they receive clean pesos in exchange.
Through these means the traffickers can avoid carrying large amounts of US currency across border or having to channel them through financial institutions, which may attract the attention of the authorities.
The US is the main market for illegal drugs manufactured in South America and smuggled through Mexico.