As usual Lonnie is "dead on". If you are purchasing any home in CT you should have a radon test done. The burden is on the buyer to order the test. Depending on the results, subsequent actions may be taken by either the buyer or the seller. As you already own the house just install the system. Legal costs will equal and exceed your added expense in short order.
My suggestion would be save yourself an expensive court battle and headache, and just spend the $800-$1200 to have a radon mitigation system installed. It's really not worth several thousand dollars in attorney fees to settle a $1000 argument.
See my link below for more info on radon.
Short of having conducted a redon test, I'm not sure how a realtor would have access to this information. Additionally, the way disclosure works is that the seller is responsible for disclosing only information they have knowledge of.
Proving someone definitely knew of this material will become your challenge. Yes, as Phil states, you can sue anyone for anything but making it stick becomes the issue. Your best advice will most assuredly come from an attorney.