MOSCOW (AP) -- Novichok, a lethal nerve agent that nearly killed a former Russian spy and his daughter in March and recently sickened another couple in England, was the product of a highly secretive Soviet chemical weapons program. Here are key highlights of the program and the agent.
WHEN WAS NOVICHOK DESIGNED?
The Soviet program to design a new generation of chemical weapons called Foliant began in the 1970s to counter the latest U.S. chemical weapons.
Soviet leaders wanted the equivalent of U.S. binary weapons -- agents made up of relatively harmless components that turn deadly when mixed, making them easier to operate than regular chemical weapons.
While agents of the Novichok class were highly lethal, the program was only partly successful, as some of the components were as toxic as the military-grade nerve agents and thus hard to handle safely.
The Soviet leadership eventually lost interest in chemical weapons, seeing them as excessive when compared with Moscow's massive nuclear arsenal. Novichok-class agents only were produced in lab quantities and never entered production. Vladimir Uglev, a top scientist in the program, estimates that about 100 kilograms (220 pounds) were made for research and military tests.
HOW LETHAL IS NOVICHOK?
Uglev, who says he was the first to synthesize A-234, a strain of the Novichok family of agents that was used in the attack that nearly killed former spy Sergei Skripal and his daughter, Yulia, described the agent as much deadlier than any U.S. equivalents.
Just a few milligrams of the odorless liquid -- the weight of a snowflake -- were enough to kill a person within minutes.
Uglev said that A-234 could remain deadly for a long time -- even if a few tiny drops are left in a syringe or impregnated into wood.
COULD IT FALL INTO THE WRONG HANDS?
The main Soviet research center that designed the Novichok-class agents was in Shikhany, a town in southwestern Russia. It was one of the "closed cities" isolated by the KGB. The sprawling facility also housed chemical depots and a military firing range, where nerve agents were tested.
Some Novichok-related research also was conducted at a main Moscow research center, which shared samples with other labs across the Soviet Union.
Despite the U.S. oversight to dismantle Russia's chemical arsenals after the Soviet collapse, Uglev said he couldn't exclude that some lab workers might have been tempted to sell toxic substances amid the economic meltdown and political turmoil in the 1990s.
IS IT POSSIBLE TO TRACE NOVICHOK'S SOURCE?
Uglev and other experts say it may never be possible to determine the nerve agent's origin.
To determine what specific lab produced a given sample of Novichok, it's necessary to find an identical specimen from the same batch -- an impossible task.
"You can identify the agent, but it's impossible to track down its source," Uglev said.
WHAT IS THE RUSSIAN OFFICIAL REACTION?
Russia has fiercely denied British accusations over the Skripals' poisoning and accused London of using the incident to fan an anti-Russian campaign. Moscow said that last year it completed the destruction of 40,000 metric tons of chemical weapons left from the Soviet era, an effort that spanned two decades under close international oversight.
Asked to comment on the second case of Novichok poisoning in England, President Vladimir Putin's spokesman, Dmitry Peskov, reaffirmed the Kremlin's staunch denial of any involvement.
"Russia has categorically denied and continues to categorically deny the possibility of any kind of involvement in what was happening there," he said.
Russian Foreign Ministry spokeswoman Maria Zakharova urged the British government to end what she described as "intrigues and games with chemical agents," and some Russian lawmakers suggested that the poisoning could be traced to a British source.