Brad Birkenfeld was a frequent trans-­Atlantic flier. He lived and worked in Switzerland, dividing his time between an apartment in Geneva and a house in Zermatt, an Alpine village at the base of the Matterhorn. But his biggest client was in California, and however gruel­ing the trip through nine time zones was, it was worth it. ­Without that client’s $200 million to manage, Birkenfeld’s position in the private-banking division of UBS would have been far less secure.

Though Birkenfeld’s job title was innocuous enough—director at the Swiss bank—his job since October 2001 was not. He helped the very rich hide tens, if not hundreds, of millions of dollars from U.S. tax authorities. He was willing to go the extra mile for his clients, so he didn’t blink when one of them asked him to do something that was blatantly illegal by any country’s standard: Buy diamonds with secret Swiss funds and bring them into the U.S. undeclared and undetected.

This would have been a challenge at any time, but in the aftermath of the September 11 terrorist attacks, getting the diamonds into the U.S. seemed nearly impossible. If Birkenfeld put the diamonds in his carry-on bag and a screener found them, he would have to exhaust his powers of persuasion concocting a story about how he had forgotten to declare hundreds of thousands of dollars in precious stones.

Yet declaring them wasn’t an option. That would have created a paper trail. Since the money that purchased the jewels was unknown to U.S. tax authorities, the diamonds had to be as well. To get them into the country, Birkenfeld had only one option. He had to smuggle them in.

So Brad Birkenfeld, a banker at one of the most prestigious institutions in global finance, began jamming his clients’ loose diamonds into a tube of toothpaste. Believing he had successfully masked the stones from security screeners, he boarded his flight. He was wrong about the disguise, though. Unlike cocaine, diamonds look exactly the same in an X-ray as they do in plain sight, and toothpaste certainly doesn’t mask that. Either the security screener wasn’t paying attention or he thought the shiny bits were very large whitening crystals.

As Birkenfeld strolled out of the airport onto U.S. soil, he had no idea how lucky he had been. Like a high-end mule, he delivered the tube of toothpaste to his client without incident. His audacious scheme had worked—until it didn’t.