OK, so this was complicated by the fact that my drugstore only carried plain Epsom salts in a 4-lb plastic bag.
On the way out, I put the unopened bag in my checked luggage. My suitcase had a cable tie closing it. When I arrived, the cable tie was missing and there was a TSA notice in my luggage.
On the return trip, I ran out of time packing and I ended up with the opened and mostly full bag in my carry-on. I put the bag in a plastic bin for the scanner, knowing it would probably throw up a red flag. Going through the bag scanner, the agent called for a 'bag check'. The agent kept saying "It showed up <color>", which I assume was a very bad sign. They swabbed the bag and then said they needed to take a sample for a chemical test. They took out a sample and ran two tests, one which came back ambiguous and one negative.
I told them I understood that this was a questionable item, and I was prepared to throw the bag away. They said they wanted to call an expert over. I asked if I was free to dispose of the bag and go to my gate or if I needed to stay, and they indicated that I needed to stay there.
They paged someone who had a badge that said 'Explosives Expert' and he ran a battery of chemical tests. In the meantime, another TSO started filling out a form and asked for my boarding pass and ID. I handed him my BP and passport, and he wanted my driver's license. Then he needed my SSN. I said, "Isn't my passport number sufficient?" and he looked puzzled/reluctant, so I went ahead and gave him my SSN.
They asked why I had the salts, and I explained that my doctor instructed me to use them. They said I was free to go and could take the bag. I asked them if they wanted to keep a sample of the salts, and they declined.
All in all, not too bad, everyone was polite, except one TSO - when I said that I was prepared to throw the salts away because they might be questionable, he asked "So you're testing us?"
I guess I'm glad they they subject 3.5 lbs of crystalline powder in a carry-on to a high level of scrutiny. I wish they hadn't requested my SSN, but I didn't really protest.
When I retrieved my checked luggage, the cable tie had been removed, but no note from the TSA. I don't know if that's significant, since I just recently started using the cable ties.
The lesson: carry a factory-sealed container in your checked luggage if at all possible.