Interesting topic Jake!
Would you interpret 'material' as meaning 'having relevance' to a crime. Would there be any qualification on that witness. For instance; he saw something relevant to the crime, overheard two people talking about what happened, was told something about how the crime was committed or a confession, like something simply overcame him and he can not even remember doing it?
I would also wonder if there would have to be some reason the person might take the 5th, and if they are held until it is possible for them to give their testimony, then they will have to respond and can not escape testifying.
if they were family, a lover who was involved with this person at the time of the murder {the affair had not been broken off formally yet}, someone who was approached to assist after the crime, someone whose personal life might be jeopardized or financially dimmed by the association with a possible murderer; these could all be people who would then feel admitting something under oath that might be detrimental to their lives. It could also mean that these people would never want to say anything that could be used against this person, as they do not want to see him incarcerated for the rest of his life or be put to death, or that they might leave the area before served with a subpoena and then would be untouchable.
I really thought in America anyone could take the 5th at any time.
Scandi