Which chemical weapon was used in Syria? Here’s what investigators know.

“The smoke was white and thick,” Hussam Salloum, a volunteer with an air raid warning service in rebel-held areas, told Reuters. “The smoke began to spread out across the town, until there was a layer.”

That description — how a bomb looked, what sound it made, whether the smoke was black or white — is one major clue in a bigger, more complicated mystery: What kind of chemical weapon was used in Syria this week? How is this attack, which killed at least 70 people, many of them children, different from those before? And will it matter to the international community?

[NOTE: This post contains graphic images.]

Experts say that Syria once had one of the largest chemical weapon stockpiles in the world. The collection, which stretched across 50 facilities around the country, likely contained more than 1,000 tons of mustard gas, sarin and VX. Some of the defining characteristics of these substances, vis a vis the BBC:

• Sulphur mustard (also known as “mustard gas”) is easiest to distinguish — it often smells like garlic, onions or mustard. It can be clear, yellow or brown. When people come in contact with the vapor, it causes blistering of the skin and mucous membranes on contact. The gas itself is not lethal, but direct exposure can result in scarring, respiratory problems and even death from pneumonia. There is no treatment or antidote, so the body has to heal the damage.
• Sarin — 20 times as deadly as cyanide — is a powerful neurotoxin. It's nearly impossible to detect, because the clear, colorless, tasteless liquid has no odor. It can evaporate and spread through the air. Within seconds of exposure, the Atlantic explained, noses run, tears form, mouths drool and vomit. Bowels and bladder empty. Next, vision blurs and chests tighten. If exposed to a high enough concentration, the victim will convulse, become paralyzed and then die within 1 to 10 minutes. A victim of the 2013 Sarin attack described the feeling this way: “It just took seconds before I lost my ability to breathe. … I felt like my chest was set on fire. My eyes were burning like hell. I wasn't able even to scream or to do anything. So I started to beat my chest really hard … to take a breath. … It was so painful. It felt like somebody was tearing up my chest with a knife made of fire.”
• VX, another neurotoxin, is the world's most potent chemical warfare agent, 10 times more toxic than sarin. The oily liquid is amber in color but is odorless and tasteless. Once in the air, people can be exposed through skin contact, eye contact or inhalation. VX poisoning looks similar to sarin, but there's a key difference — VX evaporates as slowly as motor oil. It can persist in the air for a long time under average weather, posing a long-term threat.

And this doesn't include chlorine gas, another chemical weapon that the Syrian regime has been accused of using repeatedly.

When the civil war started, Syrian President Bashar al-Assad pledged publicly that he wouldn't use these weapons against his own people. But investigators said they believed the Assad regime had been developing ways to deploy chemical weapons locally. So it was within this context that President Barack Obama laid out his 2012 red line: If Assad were to use chemical weapons, he said, the American military would intervene.

Over the next year, investigators say they believed that the Syrian government used chemical weapons “multiple” times, but it was hard to prove.

Then, in August 2013, the Assad regime dropped sarin nerve agent on the Damascus suburb of Ghouta. More than a 1,000 people died, including 400 children. The outcry was swift — Obama threatened a military response. But under pressure from a trigger-shy Congress, he worked the diplomatic angles instead. By the end of that year, Syria agreed to dismantle its chemical weapons stockpile by June 2014. In exchange, Assad and his team would face no punishment for war crimes.

Assad allowed investigators in, and Obama hailed this as a diplomatic success story. By May, national security adviser Susan E. Rice told C-SPAN that "92.5 percent of the declared chemical weapons” were out of the country. But experts were skeptical — how could the United Nations prove that Syria had surrendered its entire stockpile? And why hadn't Assad included chlorine gas on Syria's list of chemical weapons?

Indeed, by 2016, reports from investigators and inside the country suggested that Assad was “routinely” using chlorine gas against civilians in opposition held areas. (The government denies this and all other claims.) According to the New York Times:

Since then, the organization, working with the United Nations, has found that the Syrian government used chlorine gas as a weapon three times in 2014 and 2015, violating the treaty. Rebel fighters, doctors and antigovernment activists say there have been numerous other chlorine attacks, including at least two in the past week, in one case killing a doctor as he worked.

Chlorine gas attacks, though, don't tend to draw the same international attention, since they're not usually fatal. Symptoms include difficulty breathing, skin and eye irritation and headache. Fatalities normally occur only if the chlorine is released in a closed space.

On Tuesday, medics and human rights activists said they saw something different. Doctors described treating children with no visible injuries, killed in their beds. People were brought into the hospital with foam around their mouths and pinpoint pupils, a telltale symptom of nerve agents and other banned toxins. Dozens of people, including children, died — some writhing, choking, gasping or foaming at the mouth.