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They may be able to determine drowning or not through diatom testing.
When a person drowns, diatoms get into the blood stream through inspiration. Not sure if the lack of diatoms would lead them to anything conclusive, other than she did not drown.
Here's a wiki for diatoms101
From that wiki: " Samples are taken from bone marrow, lung, spleen, liver, kidney, brain tissue."
There won't be any of those left at 4 months out, sadly. Typically, they look in the actual organs but they can also take samples of those organs. Unfortunately, neither is likely to be possible in Suzanne's case, especially if her body was in water (if she was buried 6 feet under, then I'd hope that more testing could be done - but I wouldn't expect that to be a freshwater stream or lake drowning in the first place).
Diatoms and drowning
Interestingly, no diatoms would be present if the drowning were done at home. I can't remember who first suggested that idea, but it has been on my short list of possibilities ever sense. So, if Suzanne was drowned and then placed in a body of water to make it look like she drowned there, the diatoms (if present) would surely out the murderer. However, by now, it's also likely there isn't enough to test - so with each passing day, that same murderer could feel a little more confident about COD being an enigma.
Absence of diatoms in a skeleton would not provide evidence of non-drowning. I believe that at this point, searchers are looking for mostly skeletal remains. Blood and internal organs are almost certainly gone (although again, if buried deeply, maybe not - but I think that's unlikely given the soils in the area and the rather obvious excavation that would take).
Since the diatoms have to be shown to have been inside the person's circulatory system, even if a very careful dig into a grave turned up a body (and all relevant soil was preserved and studied), it would be hard to show whether the person drowned - or had merely been swimming in the diatom-infused water.
It sure would make for an interesting case. There are techniques used by some paleoarchaeologists who study paleobotany that would be convincing (a lung-shaped pattern of diatoms at the grave site) but to recover that data would in fact require a really really careful excavation, with various microscopes and test techniques in play before anything was moved. I'm not sure that has ever been done - or that even the FBI would think to use such a person. I only know of 2-3 people who do this. On a side note: those people were able to find microscopic botanical evidence of a fishing net from about 20,000 years ago - it decayed on the floor of a dwelling, was totally invisible, but these intrepid archaeologists (and I have to state that they were women) decided to look specifically for a net-shaped pattern of decayed plant material - and they found it.
That's because they had other evidence (previously overlooked by many archaeologists) of the existence of nets, which they believed were made by women. Eventually they found evidence that women were sometimes buried with their nets. Key point is that they found a net-shaped pattern of diatoms...so finding a lung-shaped pattern is possible, it's just tricky work and I'm not sure CCSO, even with the FBI, will get that done.
I do believe the body would have to be buried for any of this to occur, if stowed away in water or in a container that allowed water inside, I just don't think there will be a chance of recovering the evidence at this point.
Sorry for the long post, but it's such an interesting topic!