Diane Butrus, a business executive from St. Louis, wandered the streets of Zurich, looking for a bank that would help her keep $1.5 million hidden from America tax collectors.

One bank after another turned her down on that afternoon in 2009. They were worried about a United States crackdown on tax evasion and were no longer willing to shelter American money.

Finally, across the street from a city park, up a discreet elevator, seated in a luxurious conference room, Ms. Butrus found a banker ready to help. His name was Stefan Buck.

Mr. Buck said that his employer, Bank Frey, would be happy to take Ms. Butrus’s money, according to court documents and interviews with Mr. Buck and Ms. Butrus. He instructed her to wire the $1.5 million to Bank Frey. He told her that her name wouldn’t be attached to the new account. It would be known internally as Cardinal, an alias she chose in a nod to her favorite baseball team.

Mr. Buck, center, leaving federal court in Manhattan with his lawyers a few days after arriving in the United States in late 2016.Credit…Brendan McDermid/ReutersA Post-it note Mr. Buck handed to his lawyers after listening to Mr. Agnifilo’s stirring closing argument.Credit…Marc Agnifilo

The judge, Jed S. Rakoff, also was impressed. “I knew you were a powerful orator,” he told Mr. Agnifilo after the jury left, “but you have exceeded all bounds.”

The jury deliberated for a little more than a day. On Nov. 21, Mr. Buck was sitting on a toilet in the courthouse bathroom when the verdict came in. He hustled to the courtroom.

A pair of United States Marshals hovered at the back. “Are they here for me?” Mr. Buck recalled asking his lawyer.

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