My interpretation of the PM's words is that she implied that there is a credible evidential link. Certainly enough for the UK government to ask the Russian Federation to clarify the two questions. I'm unsure how much detail the UK government will put in the public domain. The risk is that doing so turns a legal prosecution into a public debate. My understanding—admittedly speculation—is that the nerve agent identified has a signature specific to the Russian Federation which thus far effectively rules out other possible sources. The UK government appears to have framed its questions to the Russian Federation very precisely.