If you are one of those heirs or relatives, please permit me this opportunity to express my condolences for your loss, as well as offer you some words of advice.
While your arrival at this website suggests that you are at least exploring your legal options, I'm sensitive to the fact that you are undoubtedly still grieving over your loss, and therefore may not yet be ready to take the next step. If that is the case, the last thing that I want to do is compound your anxiety by suggesting that you should do something that you're just not ready to do.
You need to remain mindful, however, that the statute of limitations in cases of this kind is two (2) years, which means that the heirs of the victim of the accident have two years from the date of that accident to file a lawsuit concerning their loved one's "survival" damages, and that the relatives of the victim have two (2) years from the date of their loved one's death to file a lawsuit concerning their own damages arising from their loss of that loved one.
In addition, you should bear in mind that with the passage of time, it can, in certain cases, become more difficult to preserve and gather the evidence necessary to properly assemble, evaluate and present the appropriate proofs pertaining to the cause of the accident, and the nature and extent of the damages.
In any event, regardless of what you choose to do or when you choose to do it, I wish you nothing but the very best that life, and the law, has to offer you.