Novichok nerve agents such as the one used on March 4 to poison a Russian former spy in Salisbury are powerful and deadly chemical compounds reportedly developed by the Soviet government in the 1970s and 1980s, and little known elsewhere, according to a chemist and criminologist at Newcastle University. British Prime Minister Theresa May says Russia was "highly likely" to be behind the attack, giving Moscow until midnight Tuesday to provide answers on the March 4 poisoning of former Russian spy Sergei Skripal and his daughter Yulia.