Yulia Skripal

The lab's chief added that it only sought to test the chemical used on Sergei Skripal and his daughter, not decide its source. Still, Russia called the remark proof the U.K. is fabricating its claims.

Tensions between Russia and the West are ratcheting up as the Russian Foreign Ministry announced the expulsion of the officials and the closure of the U.S. Consulate in St. Petersburg.

Yulia Skripal was poisoned with her father, Sergei, in an alleged nerve agent attack. While an international dispute has raged, Yulia has been progressing — but Sergei is still in critical condition.

The Metropolitan Police says the highest concentration of the rare chemical was found on Sergei Skripal's front door in Salisbury, England. Investigators assure neighbors that they are safe.