“Milord, we found microscopic blood belonging to the defendant at the scene of the crime. This proves her guilt.”
How do prosecutors prove that the evidence was actually there and not planted or the result of contamination? Is part of the original retained?
They will have the people who collected the evidence testify and show their records, as well as the technicians who performed the testing. The results of testing will have to be displayed, if the actual evidence can’t be displayed some record of the testing results may be. Just as with macroscopic evidence it’s up to the prosecutor to convince the judge or jury that the evidence was properly collected and tested. Defense attorneys can always challenge the witnesses.
I have heard of cases where evidence was disallowed by a judge because the defense was not able to perform their own tests when the evidence was consumed by the state’s testing, but IANA lawyer and can’t tell you the details of how that works.
They don’t actually PROVE anything. They just have to convince the judge and/or jury that they’re right.
The police collect the evidence, keep records of how it was collected and stored at every step along the way (chain of custody) and deliver it to a testing lab. The lab runs their test, records the results, and then the prosecutor will pay an expert to show up in court and answer questions about the test results and what it means. The defense can hire their own expert to interpret the results in a completely different way, or challenge some aspect of the original testing, or even to conduct their own tests. In some cases, the judge may be called upon to make a ruling about whether the kind of test is itself admissible. For example, in the USA, the fingerprint evidence is widely accepted but polygraph evidence is not. If the test is something new which relies on unproven technology, the judge might insist on being convinced that the test is reliable.
The sad truth is that fingerprint evidence is not nearly as reliable as everyone assumes it to be. Fingerprints are identified by looking for markers in one sample and trying to find those same markers in another sample. There is no universal standard about how many markers you have to find in order to call it a “match”. And there has never been a thorough scientific study to determine the likelihood of two people in a given population having “matching” fingerprints. The fact is that it MIGHT be true that there are two different people who both live in Chicago whose right index fingerprints are a 10-point match for each other, and one of those people could leave a print at a crime scene and the other could get arrested and convicted based on the assumption that fingerprints are unique. Certainly such a scenario is unlikely. But nobody really knows just HOW unlikely it is. Is it a hundred to one against? A million to one? Ten to one? No scientific study has ever been done to answer that. But we blindly press on, pretending that fingerprints are infallible.
Getting back to the OP, it’s certainly possible to plant evidence, and the defense might try to argue that this was the case. A police officer who was caught planting evidence would likely be fired and/or sent to jail, so that’s a strong deterrent. But it would be naive to think that planting evidence never happens.
Question having been answered, I can’t read the OP without thinking of the Lizzie Borden trial, in which blood on her clothing played such an important part… but was dismissed because the entirely male courtroom didn’t want to argue that it wasn’t m*nstr**l blood. A fascinating case of social propriety and perceptions overruling obvious evidence and conclusions, which had echoes in the first cases involving microscopic and DNA analysis of blood evidence.
I’m quite sure I have never seen anybody self-censor the word “menstrual” before.
It was meant to be humorous.
In the trial, when blood spots on the dress Borden was supposed to have been wearing were questioned, she answered, “flea bites” - the contemporary circumlocution for spotting from menstrual blood.
The trial dropped the matter like a hot turd, and the case against her fell apart from that and the underlying assumption that a woman, especially one of Good Breeding, simply couldn’t have done such a thing.
You rule out tampering by anyone other than the police, by the police testifying that their chain-of-evidence procedures adequate to prevent anyone else from being able to do it. And you rule out tampering by the police themselves by necessity: Once you allow for the possibility of the police framing someone, it becomes impossible to prosecute any case at all. Even with easily visible evidence, how do you prove, for instance, that the gun in Exhibit A was actually the one the police found on the scene?