They said there weren't any environmental factors. However, we don't know that much about brain cancers and the only real environmental factor would be radiation and some meds (like the mini pill). Checking the water supply is fine, I guess, but it takes years for the cancers to form (usually). Who knows if whatever they would look for would still be there. I think there's little chance of finding a new environmental factor. I think it's likely some unknown factors exist. I just don't think it's likely they'll find one this time.