Several days after a former Russian spy and his daughter were found catatonic on a bench in Salisbury, England, British, Prime Minister Theresa May revealed that the pair had been poisoned by a rare and highly-deadly nerve agent known as Novichok .

The revelation prompted U.S. Secretary of State Rex Tillerson to say the poisoning clearly came from Russia . Novichok, he added, is a military-grade agent found only in the hands of a very, very limited number of parties.

Heres what we know about the agent and why its been described as definitely Russian :

Novichok was developed in the Soviet Union in the 1980s

The name Novichok, which means newcomer in Russian, applies to a group of military-grade nerve agents that the Soviet Union developed in the 1980s.

A senior Soviet chemist and whistleblower Vil Mirzayanov revealed the agents existence in the mid-1990s. In a report he co-wrote for the Stimson Center , Mirzayanov described how the chemicals were developed at the State Scientific Research Institute of Organic Chemistry and Technology in Moscow during the final years of the Cold War.

At the time, the U.S. and the Soviet Union were making public pledges to dismantle their existing stockpiles of chemical weapons. Novichok, Mirzayanov said, was developed in secret and designed to be undetectable to inspectors.

According to NPR, Novichok is made with organophosphates, commercially available chemicals that are used in fertilizers and pesticides.

They possess a different chemical structure to some other nerve agents, though their effect on the human body is said to be the same.