The suspected chemical attack in northern Syria this week came more than three years after global outrage following a previous strike using toxic agents had forced the regime of President Bashar al-Assad to agree to eliminate those weapons from his arsenal.
The incident also raises again questions about what the international community can effectively do to stop such horrors from reocurring.
The latest attack
Tuesday's atrocity took place in the town of Khan Sheikhoun, where at least 72 people, many of them children, died after airstrikes. Survivors said victims showed no signs of external injuries but had trouble breathing, suffered convulsion and frothed at the mouth.
Those symptoms are "signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents," the World Health Organization said, adding that it was shipping from Turkey supplies of atropine, an antidote to nerve agents.
Médecins Sans Frontières, whose doctors treated some of the victims, went further. The medical NGO said that eight of the patients showed symptoms consistent with exposure to nerve agents such as the chemical weapon sarin or similar compounds. But MSF also said that some victims smelled of bleach, suggesting they had been exposed to chlorine.
What happened in 2013
The Khan Sheikhoun attack was reminiscent of a rocket strike in the Ghouta area of Damascus on Aug. 21, 2013, that killed hundreds of people.
A United Nations investigation later concluded that there was "clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used."
Threatened with punitive airstrikes by the United States and pressured by its Russian ally, the Assad government agreed in the fall to participate in an international treaty that would require the dismantling of its chemical arsenal.
The legal framework
Syria was already one of the states that signed the Geneva Protocol of 1925, which prohibits the use of asphyxiating, poisonous or other gases in warfare. The only reservation Syria made when it signed the protocol in 1968 was that its ratification did not imply a recognition of another party to the protocol, Israel.
Under diplomatic scrutiny, Syria agreed to become a party to the Chemical Weapons Convention, the international treaty prohibiting the production and use of toxic agent.
The convention's schedule of banned chemicals include nerve agents such as sarin and tabun. Chlorine is not listed in its schedule because it is a product with other non-lethal purposes, but the convention prohibits the use of any chemicals to cause death, incapacitation or permanent harm.
The Organisation for the Prohibition of Chemical Weapons is the global watchdog policing the application of the Chemical Weapons Convention.
Established in 1997, the OPCW is headquartered in the Netherlands. It conducts inspections and verifies the destruction of chemical-weapon stockpiles, in Syria but also in Iraq, Libya, Russia and the U.S..
The Canadian federal government says it is the OPCW's largest voluntary cash donor, contributing $22-million since 2012. Other countries have provided in-kind contributions, such as the chemical-weapon destruction facilities made available by Britain and Germany.
The OPCW's fact-finding mission in Syria has confirmed the use of chlorine and sulfur mustard as chemical weapons.
The upcoming investigation
The OPCW is gathering information about the latest incident.
It had previously provided a team of experts for the UN investigation into the 2013 Ghouta attack. That probe's methodology highlights how experts will determine what happened this week in Khan Sheikhoun.
Investigators conducted detailed, on-camera interviews with survivors, first-responders and medical personnel.
They collected blood, urine and hair samples from victims.
They also secured weather records (to reconstruct how the chemical agent might have spread) and picked up soil samples, metal fragments, rubble and used solvent-soaked wipes to look for chemical traces on floors, walls, fabric or footwear.