I'm so sorry for your loss.
Though not under the best of circumstances, welcome to BYC. I hope we can be of some help.
I agree with @Jesusfreak101 The only way to know for sure is with a necropsy. Put one of the birds in a bag and refrigerate. Then call your local ag extension office and find out where you can have one done. Some states will do it for free, some charge.
With that said, it is likely they got into something poisonous. Do you have bait out for mice or rats? Even if they can not get to the bait, if they caught some mice that ate it, it can affect them, too. If that is not possible, start looking around their coop and run area for anything they may have eaten. Chickens are very curious and can get into lots of things if they are not secured. It could also be something they drank.
Look at your feed. Make sure it is not moldy. Make sure it smells and looks good.
Please keep us advised if you find out anything.
ETA - I forgot to ask - How old are they and what conditions were they housed under? If they are chicks under a heat lamp, they may have gotten too hot. There are lots of possibilities. The more information you can give us, the more we can help.
