THE HIGHLANDER, such a shame to read about the terrible incident which resulted in the death of your dog, my sympathies are with you and I can understand your anger.
However, since I am unable to imagine that anyone could be sick enough to administer poison to the poor animal (in the circumstances you have described), I am prompted to enquire of you whether or not you received a professional opinion on the death of your pet.
If it was left in your vehicle, (which you say it was) and the temperature was a little severe, is it not possible that 'Heat Exhaustion' was the cause of it's demise?.
Under such circumstances, as above, it is possible for a dog, particularly thoroughbred, to fall victim to the effects of heat even though there may have been (what you considered to be) adequate ventilation.
Fatalities in the conditions you have described, have been known to occur within 15-20 minutes.
I am not suggesting that you were in any way negligent, perhaps by drawing your attention to these facts, other dog owners among the forum members may take note and learn from your unfortunate experience.
My sympathy and best wishes to you in any case mate.