On 10 April 2006, a DC-9 jet landed in the port city of Ciudad del Carmen, on the Gulf of Mexico, as the sun was setting. Mexican soldiers, waiting to intercept it, found 128 cases packed with 5.7 tons of cocaine, valued at $100m. But something else – more important and far-reaching – was discovered in the paper trail behind the purchase of the plane by the Sinaloa narco-trafficking cartel.

During a 22-month investigation by agents from the US Drug Enforcement Administration, the Internal Revenue Service and others, it emerged that the cocaine smugglers had bought the plane with money they had laundered through one of the biggest banks in the United States: Wachovia, now part of the giant Wells Fargo.

The authorities uncovered billions of dollars in wire transfers, traveller's cheques and cash shipments through Mexican exchanges into Wachovia accounts. Wachovia was put under immediate investigation for failing to maintain an effective anti-money laundering programme. Of special significance was that the period concerned began in 2004, which coincided with the first escalation of violence along the US-Mexico border that ignited the current drugs war.

Criminal proceedings were brought against Wachovia, though not against any individual, but the case never came to court. In March 2010, Wachovia settled the biggest action brought under the US bank secrecy act, through the US district court in Miami. Now that the year's "deferred prosecution" has expired, the bank is in effect in the clear. It paid federal authorities $110m in forfeiture, for allowing transactions later proved to be connected to drug smuggling, and incurred a $50m fine for failing to monitor cash used to ship 22 tons of cocaine.

More shocking, and more important, the bank was sanctioned for failing to apply the proper anti-laundering strictures to the transfer of $378.4bn – a sum equivalent to one-third of Mexico's gross national product – into dollar accounts from so-called casas de cambio (CDCs) in Mexico, currency exchange houses with which the bank did business.

"Wachovia's blatant disregard for our banking laws gave international cocaine cartels a virtual carte blanche to finance their operations," said Jeffrey Sloman, the federal prosecutor. Yet the total fine was less than 2% of the bank's $12.3bn profit for 2009. On 24 March 2010, Wells Fargo stock traded at $30.86 – up 1% on the week of the court settlement.

The conclusion to the case was only the tip of an iceberg, demonstrating the role of the "legal" banking sector in swilling hundreds of billions of dollars – the blood money from the murderous drug trade in Mexico and other places in the world – around their global operations, now bailed out by the taxpayer.