It will likely never be known what compound, but the "spray bottle" tells us that the agent was targeting respiratory system, not dermal. However the use of V-Series agent is a strong indicator of military assassination as almost all research on these chemicals is confined to secret military research. The Russians have admitted to the synthesis of more than 100 more novel V-series nerve agents.[1]https://en.wikipedia.org/wiki/Novichok_agent

While I'm not convinced a binary compound was needed in this case, it is an interesting theory!