I’m watching Dateline and this suspect left a police interview and committed another murder. The first cops couldn’t arrest him because their evidence was all circumstantial. While they are looking for him now, the voiceover says DNA results are still not back. At this point it’s been several weeks. So Eli5 – DNA results: wtf takes so long?
Is there only one machine in the world? Can’t they move murder cases to the front of the line? Does the DNA fairy only come once a month? Does DNA take 500 hours to marinate? Can the cops buy another DNA machine? Do we need a GoFundMe?
In: 6
Tldr; first part is that analysis takes time, even if pushed to the top of the queue. And tests need repeated to be confirmed and validated.
Second section I wrote is on dna evidence can also be circumstantial evidence. Most evidence in cases are circumstantial evidence. You just need to have overwhelming amounts of circumstantial evidence that all point to guilt to be beyond reasonable doubt.
Violent crimes and murders will take priority, but there are still thousands and thousands of samples. With the best tech, the sequencing is fast (but let’s face it, the police do not get the most uptodate machine bought for them every time a new one is brought out). However, the analysis of that sequencing info is what takes time. To say one bit of dna belongs to that person you need to compare multiple areas within the dna. You then need to repeat the tests to confirm the findings.
On top of this, just because you have dna match, it doesn’t mean that is not circumstantial evidence. And a lawyer can argue that. Depending on the case it could be explained away in different ways.
The World’s End murders in scotland happened in 1977. It was a cold case for decades and someone was tried in 2007 based on dna evidence found. The accused, Angus Sinclair claimed he’d had consensual sex with the victims and that any harm that came to them, happened after. So the dna was circumstantial evidence. He was acquitted
This was the case that got Double Jeopardy laws in Scotland over turned, and the first case to be retried. Sinclair was retried in 2014 further forensic evidence had been discovered (had not been looked for previously as the prosecutors thought the dna was enough evidence, but that’s a whole other issue). He was convicted then and sentences to life. Dies in prison in 2019.
Most cases use circumstantial evidence, it’s just when you have more and more pieces of evidence adding up that you can prosecute. Fingerprints at the scene, could be there because the suspect did it, or were there before the crime occurred. Fingerprints on the weapon; defence are not disputing that the weapon belongs to the suspect, just that someone else used it for the murder. Semen present, consensual sex. Video evidence showing the suspect in the area at time of the crime, doesn’t mean they did the crime. But add them plus other things up, is how you get the case.
So I’m your case, they were waiting for a further bit of circumstantial evidence. Probably would have been enough to have him off the streets, but it doesn’t mean that there weren’t other things that could be looked for either.
I’m UK based, but I’ve read that the forensic services in the UK and the USA are often underfunded, can’t hire/retain enough staff and have huge backlogs of work.
I’ve also read news reports of situations in the USA where crime scene specimens weren’t collected or stored properly so the material gets contaminated or degrades to being useless.
I’m not sure why that is the case in the USA, but in the UK it has been attributed to cost cutting measures under the guise of “streamlining the service” resulting in lower salaries (compared to other similar qualified work) and centralisation of forensic services leading to massive backlogs of work.
ELI5: It comes down to a lot more demand for testing than available testers. It also is a time consuming process which takes around nine hours per batch. Resource allocation may also be a factor, which like you infer, sends the most severe cases to the front of the line.
Source: am cop who submits DNA swabs regularly on cases. Obviously this is a very generic comment. Actual times and reasons may differ across the United States and other countries.
An ELI5 regarding why the reality of how long DNA takes seems unreasonable: every crime procedural drama on TV makes it look like DNA evidence only takes minutes or hours to process. Like every other aspect of real life crime investigations and trials, there’s a lot of waiting involved in analyzing evidence. That’s not interesting TV, it’s much more dramatic to show crimes being solved in a day.
That has actually affected how juries perceive the evidence presented in court – they can be suspicious of major crimes cases like rape and murder if the prosecution doesn’t have DNA evidence because they’re used to seeing it a lot on TV. They also assume it’s infallible. It’s similar to jurors’ preference for eyewitness testimony even though it’s one of the most unreliable forms of evidence.
As others have commented the government labs are underfunded and not the newest technology. The people certified to handle DNA evidence are also limited. The protocols required to ensure a sample is not contaminated are insanely strict, just breathing near the sample could contaminate it.
So taking into consideration the insane handing procedures you also have to clean between each sample and step. Processing a sample isn’t like in CSI where they just chop off the qtip into a vial, vortex it and shove it in a machine. There’s several steps that are used to clean and prepare and purify the DNA. There’s a bunch of stuff in your blood or cells that isn’t DNA, and all of that needs to be cleared away so the DNA sequencing can occur.
Then it’s possible that labs use next generation sequencing, which you can run just the full DNA as it is. But more likely they need to do a PCR step to replicate and amplify a certain portion of a DNA strand for comparison. Next gen is over 500$ a sample and takes half a day to run on a half a million dollar machine. PCR and regular sequencing would be 2-3 days of processing on machines that are 20-100k and only maybe 20$ a sample.
Every sample needs to be rerun to verify with a handful of standards and checks to verify the results. Once you run on the machine it spits out a bunch of meta data that then also needs to be processed. I’m not sure how they do it with human DNA sequences but I’ve had to do bioinformatics on environmental microbial samples. That step alone can take hours to days to analyse.
So in summary, cost, technology, limited resources and sheer volume of samples.
For starters, a geneological DNA test can take 4-8 weeks to process from the time they receive your sample. However, the police are often working with trace samples which are harder to anaylse than cheek swabs, and since you could potentially send an innocent person to the chair if you mess up, the testing standards are much higher.
Ok, forensic DNA analyst here and so far none of these answers are correct. The answer is batching.
We could conceivably process 1 small case in 24 hours. So beginning to end, you can have your results tested, written and reviewed and back tomorrow.
But we have 100s of cases. A few come in every day. So we can have this awesome turnaround time for a few cases and also have this horrendous backlog or:
We can do 100s samples at a time over and over again, pushing them through the process together. So we can get you dozens of cases done in a few weeks, but none in 24 hours, and also no backlog.
It’s about batching. If you batch samples together they overall get done faster. If you have a serial rapist, or a missing child, of course, let’s push those through today on their own and get them done, but understand that when you prioritize cases like this, you’re pushing back other rapes and murders from being tested so push cases ahead thoughtfully.
Edit: Adding that murders don’t generally get pushed up the list because most times, someone already knows who did it and why. Most murders aren’t random. Rapes are random, though not always. Kidnappings can be random. Murder, rarely.
DNA sequencing can be done very quickly now, but if you have TONS of samples to get through, it’s a lot more efficient to run them in batches as others have mentioned. This takes a little longer, but you can get through a lot more samples in a reasonable amount of time. They can probably rush the results if it’s super urgent for some reason, but generally a few days vs. a couple weeks doesn’t make a huge difference for drawn out murder cases.
Latest Answers