That kind of evidence may have traction in the court of public opinion, but it usually doesn't get to first base in a court of law.
However, the law, in a murder case, as been tweaked by Illinois lawmakers largely because of drew Peterson, and now, the changes could be used to prosecute him.
Before her death in 2004, Kathleen Savio sent a letter to a prosecutor. It said, in part, that Peterson "knows how to manipulate the system, and his next step to take my children away. Or kill me instead."
A statement like that would typically be considered hearsay, which means it would not be admissible in court. But after the change in state law, it could be allowed.
"In essence, what you're allowing the victim to do is testify from the grave," said Will County State's Attorney James Glascow.
To use Savio's statement, before the actual trial begins, prosecutors will go before the trial judge and try to convince him that:
"A: Peterson killed his wife, and B: That he killed her in part to keep her quiet, to stop her from testifying, which is a high hurdle," John Marshall law school Prof. Justin Schwartz told ABC7 Chicago.
If the judge concludes that is more likely true than not, he can allow it.
"It is a strange and not necessarily welcome development in the hearsay rule because it opens up the opportunity for lots of potentially unreliable statements to come into a criminal case," Atty. Lori Lightfoot said.
There is little doubt that the new law will be appealed, in part on constitutional grounds that the accused has a right to confront witnesses against him.
Law professor Leonard Cavise believes that the Peterson case might be allowed to proceed on the so-called hearsay evidence, but that it ultimately won't pass constitutional muster given what the U.S. Supreme Court has already ruled in a similar case.
"I'm looking for the trial judge who'll say, 'I've read the U.S. Supreme Court's opinions, and I understand this is unconstitutional. This is a denial of the right to confrontation, and I'm not letting it in,'' Cavise said.
"We have a rough road ahead of us, but I believe we can meet that burden," Glasgow said.
When prosecutors have a pre-trial hearing with the judge, they will ,no doubt, have evidence to offer beyond just the past Savio letter to the prosecutor. However, they are not disclosing that evidence. If they have, for instance, a collection of potentially incriminating statements, the judge must decide if they satisfy a preponderance of the evidence that Peterson killed Savio and he did so to prevent her from eventually testifying against him.