Have you just watered, or have you been checking for soil moisture?

Alabamanicole, thanks for your question, yes, all my watering of these containers produced good soil moisture throughout. The soil is not impacted. I am suspecting something has impacted the root structures on this particular container...first thought was possibly an ant colony. I will have to dump it out and see and re-plant, I think. No saving it now...as of this morning, all the plant material is pretty much beyond hope.

I was thinking it might be over watering, not under watering. But if they are definitely dead, and examination of the roots might help.