The case of Vladimir Kara-Murza leads to a simple diagnosis : a first poisoning in 2015 with a mix of chemicals and alpha-emitting nanoparticulates, and a more limited chemical attack in 2017 that was enough to create significant damage because of latent weakness, as the first pack of alpha-emitting nanoparticulates had obviously not been epurated entirely.
The chemical that could have been used was certainly not citalopram. A product like sarin, to me, is likely.
I think the Lawrence Livermore National Lab simply opposed getting samples that had already been touched by external hands. This simply for reasons of methodology. But this quote from the RFE/RL article leaves no doubt : Another doctor said that Kara-Murza appeared to have suffered from an inflammation of blood vessels in the brain or spinal cord, something known as central-nervous-system vasculitis. The timing was potentially suspicious, the doctor noted: “Given this developed all after his acute illness possibly propagated by a toxin/poison, it seems likely a secondary phenomenon” rather than a standalone disease.