There isn't much 311 can do about this. You should call Pct. 7 Constables (713-643-6602), who MMD pays to patrol Midtown, and report it to them along with the photo of the offender. Depending on the threat level and his ability to carry it out, I doubt there is much to prosecute him for though, maybe littering.

We're all dying to great what the threatening message said.

You'll just have to wait and legally place your heavy trash in the neighborhood near that strange individuals house I suppose...

I really don't think threatening messages from street people should be a joking matter. Some of them are just crazy enough to carry them out.
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Threatening message is not for 311. That is 911. Dumping trash can be charged as a class B dumping misdemeanor. Contact HPD.