Sergei Skripal, a former double intelligence agent for Russia and Britain, was taken critically ill after being exposed to a mystery substance.
He and his daughter Yulia collapsed on a bench in Salisbury, south England, on Sunday. They remain in critical condition.
The case has sparked wild speculation that Skripal was targeted by Russian assassins, but the Kremlin has denied any knowledge.
In a mark of how serious the matter is being taken by UK authorities, counterterrorism police are now leading the investigation into the apparent poisoning.
Scroll down to learn more about Skripal, and see exactly how he went from working for Russian intelligence to fighting for his life at a hospital in Britain.