"You have to prove that [a defendant] knew what the law was and willfully violated it anyway," said Jay Nanavati, a former tax prosecutor at the Justice Department, now in private practice. He said that merely proving wrongdoing was not enough: A defendant "can say, 'I did all this stuff. I meant to do all this stuff. And either I didn't know it was illegal, or a [lawyer] told me it was OK.' "