“There is no doubt about the use of sarin,” French Foreign Minister Jean-Marc Ayrault told reporters. “The responsibility of the Syrian regime can no longer be doubted.”

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The Assad government did not comment directly on the French report, but has previously denied responsibility for the attack in early April. A deal brokered in 2013 by Russia and the United States called for the destruction of all Syria's chemical stockpile.

The French report also offers a window into how researchers can make assessments on the use of chemical weapons, particularly when they can't visit the site in person. That's the case in Syria, where few foreigners travel freely.

France said its researchers used evidence such as blood samples from exposed to the nerve agent in Khan Sheikhoun as well as analysis from items carried from the village. The report said it found hexamine, a hallmark of sarin produced by the Syrian government.

“In situations where we can't get on the ground, we have to rely on our network on contacts inside the country,” says Lama Fakih, a deputy director in Human Rights Watch's Middle East and North Africa division. The organization will soon release its own report on the attacks.

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In practice, that means a couple of things: reaching out to a wide variety of people in Syria, including witnesses of the strike and those who might have been close by. Fakih says that her researchers ask each source to recount what they saw, and they press on little details. What color was the smoke? What sound did the bomb make? Did they smell or taste anything unusual?

Everything, Fakih says, is verified by a couple of different sources. The information is then brought to weapons experts, who use these clues to figure out what kind of weapon was used. “We try to reach out to as many people as possible,” Fakih said. “Then we really do interrogate what they're saying to see if it stands up.”

For example, if someone tells her investigators, I saw the missiles fall, they'll ask where they were standing and figure out whether that would have actually been feasible. And they always ask if there's anyone else who can confirm their story.

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Her researchers also talk to victims of the attack itself, to understand exactly what they experienced. What, they ask, were their first symptoms? What came next? How did they look as they were experiencing them? They ask the same of doctors and nurses. This list of symptoms helps experts suss out what kind of chemicals were used. When people are exposed to sarin gas, for example, their pupils shrink to little pinpricks.

>There are other ways of corroborating, too. Human Rights Watch researchers ask their sources for video and photos. They compare satellite imagery of the affected area before and after the attack. Their goal, in doing so, is to weigh every possible theory.

“We definitely look toward all sort of alternative theories, try to play it out,” Fakih said. We ask, “if it's from another party, how would it have been delivered and what's the proof of that?”

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For example, the Russian and Syrian governments claimed that the chemical weapons were released by an attack on a warehouse by rebel groups. So Human Rights Watch researchers investigated. First, they looked for a warehouse that could have been hit. Once they identified possibilities, they consulted satellite imagery to see if any damage had been done. In this case, they did identify one old grain silo that had been struck between February and April.

Next, they talked to community members to better understand whether that structure could have held chemical weapons. Locals told them that this was impossible. The warehouse was missing some walls, and it hadn't been used as a warehouse in years. Neighbors used it to play volleyball. Researchers verified that story with photos and video of people playing. They also consulted a video of the explosion to see if there had been a secondary explosion, which you'd expect is an ammunition depot had been hit. They didn't find one.

To determine responsibility, investigators also look at the weapons. For example, multiple witnesses told Human Rights Watch that the gas was dropped by a warplane. Only the Syrian army has access to planes. Researchers also know that the Syrian government has access to sarin gas.

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Human Rights Watch has another goal. They want to understand the impact of these attacks: How do they effect the victims and their communities?

“Accountability is often at the forefront of what they want to see happen,” Fakih said.