Thousands of people have been fatally gassed with nerve agents on battlefields since World War II, most recently in Iraq and Syria, but they are not a weapon of choice for urban assassinations.
Sergei Skripal, a 66-year-old former Russian double agent, and his 33-year-old daughter Yulia were poisoned with a rare toxin on Sunday at a shopping mall in Salisbury, southern England, and remain critically ill in hospital, police said.
Investigators said they had identified the specific agent, but were not disclosing that information.