There are several members on this thread that experienced a similar situation called cocci. When chicks quickly decline & die in about a day it's a likely cause. It's very common in chicks because adult chickens build up an immunity. One symptom is chicks looking lethargic & fluffed up like they're cold. There's also runny poop & often blood in poop. The good news is that a med. called Corid & a good brooder cleaning can help save lives - if it's cocci. Putting it in their water at the 1st sign of cocci symptoms also increases survival. Because of all the sad stories I've read, I keep a package in my chicken 1st aid kit. (It saves time & a long drive when you want to start treating ASAP.) I can't say for sure what killed your chicks but it's something to research.
Also look at the environment. Could they be eating something strange like shavings, styrofoam? Judging by their actions, are they the right temp? (Huddled in the warmest spot = cold, spread out far away from heat source = too hot) Are they getting stressed out by a pet or young child?
I actually bought some corrid when we got our first ones. So I can add it in instead of electrolytes. I havent seen any bloody poops in there. I did clean the brooder out, we just have the one left now so do I get new ones now or wait and see how this one does alone? I did notice that she was bigger than the other one when we put her in. I am very worried about her being alone for too long. I also still gotta find where to get new chicks. The lake barrington place only had 3 week olds.
The brooder is about 90 under the light, they were laying about halfway under it. Not to the edge but not right under the light. I didnt see them eating anything weird. They are on shavings, but i didnt see them going after it. Nothing else in there for them to get at really.