STEVE BIERFELDT had a little tin box, packed in plain view in his carry-on bag, when he went through airport security in St. Louis last March. He often carried the little box, he later told me. But this time, the box would become a big deal.

Security screeners had a problem, not with the box but with the cash inside it, about $4,700. They demanded he explain where he got it. Trouble ensued when he politely declined to do so. He thought he was within his rights, he told me.

“Am I being detained?” he asked after the regular checkpoint security search had been completed.

Yes, he was being detained.

Transportation Security Administration officers escorted Mr. Bierfeldt to a nearby interview room, where he was interrogated for more than 20 minutes by screeners and then by the police officers who were soon summoned.