It kills as a chemical, but is detectable as a radioactive substance. Using it means you have access to a nuclear reactor and extensive infrastructure. It is a weapon that could only have come from a state actor, one with an extensive nuclear program and a willingness to weaponize material from that program. That is a very short list. Had they wanted to remain anonymous they could have used any of a dozen different methods that could not be so easily traced. They weren't trying for a knife in the dark.