You might consider opening the hive and examining the brood combs. If you can find any remaining cells of brood, take some closeup, in focus photos. Is there any honey left in the combs? Examine the dead bees, carefully, on the bottom board...see any mites?
Doubtful the colonies were poisoned this time of year. Could be starvation, but more likely a varroa issue. Did you ever sample for mite population? Did you treat for mites to reduce their population in your hives? What did you use and when?