The U.N. has accused the Assad regime of repeatedly committing war crimes in Syria. Including a chemical attack in April on a town called Douma. It killed dozens of people and triggered U.S.-led strikes inside Syria. To this day, Syrian officials and their Russian allies deny the attack ever took place. Finding out what really happened matters. Entire families were killed, and the regime went to great lengths to conceal the evidence. Our investigation is the most detailed reconstruction of the attack so far. We analyzed a trove of videos and interviewed dozens of witnesses and experts. We scoured some evidence with the investigative group Bellingcat, and we teamed up with the agency Forensic Architecture to create a virtual model of the crime scene. We center on one building that confirms where the attack happened. One bomb, that shows who carried it out and when. And the victims, whose symptoms tell us what happened: a lethal chemical attack. The evidence combined exposes Syria and Russia’s lies. “Where is your concrete evidence about what happened?” The concrete evidence is right here. First, it’s important to understand why Douma was a target that day. The Syrian military had launched a brutal campaign to retake the eastern suburbs of Damascus held by rebels for five years. The strategy: to terrorize the population into submission. It worked. The regime began gradually seizing town after town and Douma was the last enclave holding out. We’re going to focus on this one street. Ambulances used it to reach an underground tunnel that linked up with Douma’s last functioning hospital. And the street may have been targeted for that reason. In just two days, the bombing leveled entire buildings, one after the other. And then, on this apartment building a chemical weapon was dropped. Assad has offered two explanations for the attack. “Either the terrorists had chemical weapons and they use it intentionally. Or there was no attack at all.” Initial investigations became nearly impossible because his forces took control of Douma and the crime scene. Many witnesses and media activists were bused out. The first outsiders allowed to visit the site were Russian media, who broadcast their own distorted version of events. But that propaganda unwittingly gave us visual evidence to find the truth of what happened. We couldn’t visit the site. But we worked with Forensic Architecture to reconstruct the crime scene and study the clues it contained. And here is where the trail of evidence begins. We confirmed it’s where the bomb fell by geolocating this mosque, this school, and apartment block seen from the building’s balcony. And we made a model of the building by analyzing the videos taken inside soon after the attack. And mapping architectural details as first responders moved from floor to floor and into rooms. Gruesome as the videos were, they confirmed these bodies were found at the building that was hit. We counted 34 victims. Men, women and children spread across two floors and in the stairwell. Now we look at the bomb. Its rigging and casing tell us who dropped it. Frost tells us when it happened. And black residue indicates chlorine was used. The bomb itself is a crude chemical weapon, a pressurized chlorine tank designed to blow open on impact. And this debris tells us it was rigged to fall from the sky. This matters because Syria controls the airspace, and opposition fighters don’t have aircraft. Scattered around are small wheels and axles. And here, in the tangled remains, we see straps, another wheel and fins. When we reassemble this twisted metal in our model of the balcony it becomes a cylindrical rigging and it fits right onto the canister. This is almost identical to the rigging on a second bomb found in Douma that day. Straps and hooks so it can attach to a helicopter’s release system. Rear fins to create drag and stabilize the bomb as it falls. And small axles and wheels so the bomb alternatively can be rolled out of a helicopter. Clearly, the bomb on the balcony was designed to drop from the sky. And the damage to the casing tells us that it did. These deep dents mean the tip of the bomb made first impact, consistent with it falling nose down from above and piercing the ceiling. But most importantly, this imprint revealed on the underside of the canister. Lying beside it is a crumpled metal lattice that we think covered the balcony. Its dimensions precisely fit the pattern seared onto the canister. Our analysis suggests that this imprint was made by the force of impact when the bomb crashed through the metal lattice and onto the concrete below. Our conclusion is it had to drop from the sky. We have evidence of regime helicopters dropping them all across Syria during this conflict. In the campaign to retake Aleppo. In this attack on a hospital in Al-Lataminah. In Saraqib, just two months before the attack on Douma. And on many more occasions. Flight observers reported Syrian helicopters leaving this military airbase on the night of the attack. It was the staging point for many bombing runs that day. Soon after, the helicopters were seen circling close to Douma. This was around the time that medics and activists in Douma began reporting the attack on Twitter and WhatsApp. We verified they were in Douma, and the details match other witness interviews we took. Now, the evidence that the bomb was also dropped around this time. This video was filmed by one of the first people to enter the building after the attack. His camera light catches a white object. We know it’s the bomb because our model of the building shows it’s in the precise position that the bomb landed. It even matches the shape and contour of the canister. But what about the color? The white is a layer of frost that experts tell us coats the underside of a tank when it empties and the liquid left inside cools. They call it auto-refrigeration, a phenomenon we’ve seen in previous chlorine attacks. The frost supports the thesis that chlorine was released quickly and recently. This video was shot around 10 p.m. We know because we tracked down the cameraman, who sent us the video file data from his cellphone. Experts told us the frost can remain for hours. And this lines up with the time frame given in witness accounts of that night. The last clue on the bomb is this black residue. A metals expert told us that when chlorine mixes with water, or in this case, frost, it forms an acid that corrodes the metal creating a dark compound. So where we once saw frost, we now see black corrosion. It’s further evidence that this was a chlorine attack. So far, this collection of evidence helps us understand where the attack happened, when it happened, who did it, and the chemical that was used. Our final and perhaps most damning evidence are the videos of the victims that we verified were taken at the building. Filmed by medics and media activists, they show that this was a lethal chemical attack. We weren’t qualified to diagnose the symptoms, so we enlisted the help of experts with decades of experience in chemical weapons exposure. A warning: This raw footage is very graphic. We’ve obscured it to focus on specific details. Some experts identified two clear signs of exposure to chlorine. “Chlorine gas actually turns into an acid and the tissues produce a lot of mucus. It’s often referred to as a frothy mucus.” “There appear to be corneal burns the eyes appear almost whitened. This could happen from exposure to a chlorine gas.” We have reporting that the chlorine exposure here was extremely high. A U.S. official told us chlorine and the nerve agent sarin were found in blood samples taken from Douma. So a second chemical may also have been used. But the experts all agree that these victims were not killed by conventional weapons. And the scenes were not staged, as Syrian officials have claimed. “You don’t really see evidence of physical trauma from blast injuries or shrapnel injuries or gunshot wounds.” Lastly, the locations of the victims suggest what happened inside. Chlorine gas is twice as heavy as air and sinks instead of rising. After years of war, many Syrians know exactly what to do if they smell it. That helps explain why many of the victims were found near water sources and on the staircase. Sheltering in the basement from the intense bombardment, they moved upstairs once they smelled chlorine. They couldn’t have known it was coming from the roof or that they were moving into a death trap. The Douma attack was not an isolated incident. It’s just the latest in a string of chemical attacks that date back to 2013. And leaked material we obtained from the U.N. has implicated the Syrian regime in six attacks in 2018 alone. Taken as a whole, they represent what we believe is a deliberate strategy of Bashar al-Assad to poison his own people. These are war crimes. But the prime suspect continues to roam freely.