What if DNA or other indirect evidence point to a person being guilty, but there are no probative witnesses?
Probative witnesses are witnesses who have direct knowledge of the defendant's guilt of the accused crime. A legal investigation may collect information from many "witnesses", who testify to facts which are pertinent to a legal case/investigation, but do not have direct knowledge of the defendant's guilt.
If, for example, Witness #1 (falsely) tries to establish an alibi for the defendant ("he was in such-and-such a place at this time"), and Witness #2 testifies that she did not see the defendant there at that time, this is pertinent to the overall investigation by the judge, but the judge could not call upon Witness #2 to throw the first stone in the case of the defendant. Witness #2 is not a probative witness, merely a supporting witness, in the case of the defendent. Probative witnesses -- the ones who can be called upon to "throw the first stones" -- must be certain beyond reasonable doubt based upon their own sensory input (rather than, for example, hearsay) such that they are willing to become a murderer if they were mistaken or misinterpreted what they saw.All witnesses are subject to the death penalty for false witness in a death penalty case, regardless of whether they are probative witnesses.
DNA (and other forensic) evidence can be used by the judge to challenge or support actual probative witness testimony, but cannot count as one of the "two or more witnesses" (Deut. 19:15) for the purpose of establishing guilt. If there are no probative witnesses, the case must be dropped. DNA cannot itself be a probative witness, because the probative witnesses must be able to throw the first stones (or the modern equivalent): Deut. 17:6-7, John 8:7.
Some people have suggested that a "DNA technician" might act as the "stone-throwing" witness. But a stone-throwing witness has to be so certain of the guilt of the accused that he/she is willing to risk murdering an innocent man if he/she is wrong. DNA is indirect (sometimes called "circumstantial") evidence. DNA evidence can be planted, and it often can only show that someone, or a near relative (such as a brother), might be implicated. The technician must be able to testify with utter certainty that the DNA could not have been planted. And the technician must be able to convince the judge of this, and be able to back up this accusation by actually throwing the first stone.
That doesn't mean that everyone in a community is required to trust a person who is under suspicion -- just that they aren't allowed to use force against him in retribution, according to Biblical law.