AMSTERDAM (Reuters) - Thousands of people have been fatally gassed with nerve agents on battlefields since World War Two, most recently in Iraq and Syria, but they are not a weapon of choice for urban assassinations.

The forensic tent, covering the bench where Sergei Skripal and his daughter Yulia were found, is repositioned by officials in protective suits in the centre of Salisbury, Britain, March 8, 2018. REUTERS/Peter Nicholls

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.

The attack had the hallmarks of a well-planned hit with the backing of a powerful organization aiming to intimidate opponents, not a lone actor, experts said in interviews with Reuters.

Nerve agents have rarely been used outside of warfare.(For a graphic of how a nerve agent works click tmsnrt.rs/2FjA6EQ)

The most recent example was the assassination with VX of Kim Jong Nam, the North Korean leader’s half-brother, in the Kuala Lumpur airport in February 2017. Sarin, in a dilute form, was released in the Tokyo subway in 1995, killing 12.

Manufacturing VX or sarin requires sophisticated chemical processing, specialized equipment, access to precursors and knowledge of how to carry out an attack.

Obtaining the raw ingredients would not be too difficult, but producing nerve agents, also known as organophosphorus compounds, must be carried out in a controlled environment, not a crude basement lab, the experts said.

The final product, a highly toxic, colorless and odorless liquid, will kill whomever it comes into contact with, exposing the attacker to serious risk, either through inhalation, ingestion, or skin contact.

If exposed to a powerful dose, a victim will typically die from asphyxiation or cardiac arrest within minutes due to a loss of control of the respiratory system and other muscle functions. Those who survive frequently suffer long-term neurological damage.

AEROSOL SPRAY

Based on police reports of the Salisbury incident, experts said it is most likely that the toxin was delivered with an aerosol spray or a wipe, and transferred to the skin or clothing of the victims. They in turn probably exposed a police officer attending the incident who developed less serious symptoms.

In addition to a wide range of documented nerve agents, there are less well-known varieties, such as tabun and soman. Experts said it should not be ruled out that a sophisticated, less known variety was used in Salisbury.

“If the substance is not a well-known one, or even if it is a known nerve agent like VX, there has to be high technology behind it,” said Ake Sellstrom, a Swedish scientist who worked on United Nations missions to dismantle chemical weapons programs in Iraq and Syria.

While it is possible for a well-trained assailant to carry out such an attack, it is highly unlikely that a lone-wolf could have done it. In any case, it would be more useful to try and trace the origin of the nerve agent.

“I would look more towards the source: where did the stuff come from?” Sellstrom said.

The Salisbury attack indicates the use of sophisticated chemistry in production of the nerve agent, suggesting the involvement of a state actor, experts said.

Andrea Sella, a professor of inorganic chemistry at University College London, backed Sellstrom’s assessment.

“A key forensic issue here is to identify impurities and residues that might provide clues as to the precise chemical process used to manufacture the material,” she said.

“There is no question that the authorities will be looking for the container used to deliver the material as the chemical contents would be a goldmine,” Sella said.

Identifying precise chemical markers would be a big step toward pinpointing the source.

There has been speculation that the Russian state, even though it completed the destruction of its declared chemical weapons stockpile last year, could still be behind the attack - a suggestion dismissed by Moscow.

Any use of chemicals as a weapon is banned under the 1997 Chemical Weapons Convention and this would include the Salisbury attack. The Salisbury attack is expected to be raised at the Hague-based Organisation for the Prohibition of Chemical Weapons, a diplomatic source said.

Speaking in Brussels on Thursday, OPCW leader Ahmet Uzumcu did not specifically mention Salisbury, but he said anyone who violates the treaty should be held accountable.

“And let me be clear – under the CWC, any toxic chemical used for hostile purposes is unequivocally a chemical weapon,“ he said.