Deduction.

Skripal was picked up in POTUS FISA warrant on Christopher Steele and the UK didn't know this until after he was poisoned by MI6.

When it comes out in the FISA DECLAS that Skripal was a loose end for the UK in their act of war on the US, a storm will unfold, hence the joint effort to assassinate POTUS after G7.

PM May is toast.