Temps are fine. There doesn't appear to be any sign of actual illness, because nothing to my knowledge should kill that quickly. There should conceivably be some sort of outward symptoms. Its only the under 1 month old's that are dying off. They're all currently in the barn in separate brooders...
I bought some EE chicks at an auction on Saturday. Most appeared in decent shape, some were missing a few feathers, however all seemed active and happy, no outward signs of any illness, clear nostrils, clear eyes, energetic and frisky. They ate and drank like starving lions when they got home...