Similar to if we had a sudden large radiation hazard and it was unclear if it was some spontaneously emerging phenomenon with essentially no direct human cause, or a power plant/enrichment facility/warhead manufacturer/etc.
And beyond that, there are supposedly methods of (even purely scientifically motivated) research that would involve engineering a virus with certain properties that also happen to make it extraordinarily deadly.
Ethics committees determine all sorts of experiments to be prohibitively unethical, and a virus with particularly deadly design should certainly be held under such scrutiny.