>and no one here knows what would happen if John Smith Anglosaxon set off the same detector in the same situation.
We don't have to have direct knowledge of the impossibly specific situation you've constructed. The FBI agent flat out said that it was about his race when he said "a person of your… background".
And explosive detectors are not really there to find bombs. They are there to reduce manual searches. If they just wanted to find bombs, they could just search everyone. Since that is impractical on multiple counts, the explosive detector is valuable because it narrows down the space. Once you have taken the person aside, searched them, searched their stuff, and come up with no additional evidence ("brown" isn't evidence), it's time to let them go.
Finally, this isn't just about racism, but is also about the TSA and the cops being dangerously incompetent bullies. Instead of being polite and decent, they were rude and intimidating. Someone being reluctant and apprehensive under those circumstances is not strong evidence of wrongdoing.
And when they questioned him about the chemicals he had handled, they wouldn't even tell him what they had found. That's the kind of "withholding information" tactic you would use if you were playing the role of a detective without actually having any idea how to investigate. What purpose does it serve? If he were a terrorist, he'd know what chemicals were in his explosives. The only situation where that information is actually a secret is the one where the suspect is innocent.
In other words, they repeatedly created situations that either made guilt indistinguishable from innocence, or where they'd distinguish them inversely. This is, to put it mildly, "poor form".