This question has many layers, and you do not provide enough information to give a specific answer.
There is no statute of limitations on the State prosecution of first-degree intentional homicide. Assisted suicide, on the other hand, is probably subject to the normal six year felony statute of limitations.
If there is a credible witness with evidence that a serious crime has been committed, why is that person not going to the police? If there is an answer to that question that does not impact negatively on the credibility of that witness, then the answer to your question is dependent on the details, and turns on when the cause of action accrued.
In Wisconsin, the statute of limitations on wrongful death is three years from accrual [s. 895.54(2)]. The question then becomes when did the cause of action accrue, which can only be answered by comparing the specifics of this case to the case law on accrual.
However, if this is viewed as a medical malpractice case, the statute of limitations is three years or within one year from the date the injury was or should have been discovered, whichever is later.
You should consult with a personal injury law firm that is capable of analyzing the potential case from both wrongful death and medical malpractice perspectives, depending upon the facts.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.
The answer to this question depends upon whether you are referring to a criminal case, a medical negligence case, or a intentional tort case. I suggest you contact a local Wisconsin attorney and that you provide that attorney with all the facts. Good Luck
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