On Monday, April 15, Shelton arrived at her friends’ house in the Ashrafieh section of Aleppo. At the time, Ashrafieh was under the control of a Kurdish militia known as the Popular Protection Unit, or YPG, run by the Kurdistan Workers’ Party (PKK), a militant Kurdish organization founded in 1984. Her friends had news: just the day before, a chemical weapons attack had killed people in the neighboring suburb of Sheikh Maqsoud. “I used to always hear what’s going on,” comments Shelton. “Everybody talks about everything that’s happened lately. It’s quite easy when you’re living there or spending a lot of time there to just come across these things.”

The news was startling. “In Syria at that time, because chemical attacks were this red line, anything that remotely looked as if it could involve any kind of chemical would be labeled instantly as a chemical attack,” recalls Shelton. Still, if she could determine that this was the kind of chemical attack President Obama meant when he drew his red line, that would be a world-changing story. “This wasn’t the first one I’d checked out,” she says. But “this was the biggest [rumor] I’d come across to date.”

So on Tuesday, April 16, she walked to Sheikh Maqsoud; it took about 20 minutes. “I went in there thinking this is huge,” she remembers. “I definitely thought” it was a chemical weapons attack. She went to the Kurdish-controlled police station, where officials confirmed that there had been an attack early Sunday morning and that at least three people had died. She asked whether anyone had taken pictures and they named local journalist Rojhit Azad, who agreed to meet Shelton at the station.

Azad brought Shelton both videos and still photos. The graphic video showed the corpses of two dead children and a woman, and other victims lying on metal tables and foaming at the mouth. The photos showed the courtyard staircase of a private family home, and included shots of an explosive device witnesses said was dropped from a helicopter. As usual, Shelton was cautious about accepting the visual material. “You always have to be really careful, especially if you’re not on the ground,” she says.

You can’t verify these sorts of things. Because people, all the time, they will show you footage on their phone and go, this is what I took. I saw this. And then you see if on YouTube, posted by 20 other people.

“Journalist” was also a relative term in Syria. Once the conflict started, many individuals had started to call themselves journalists; rebel groups frequently created their own loosely edited “news” websites. In this case, however, the police confirmed that the photographer was one of the first on the scene. Once she got there, Shelton also was able to see that the photos matched the site. Finally, she noted that the journalist had been working for a couple of years already, and was in his 30s, unlike some of the younger and less reliable activists she had encountered. Too often, she says, “they just repeat stories, especially when you’re a journalist.”

Especially with chemical weapons. Because that was a red line, everybody wanted to prove that this is chemical weapons, because then maybe the US would come in and help them. Especially rebel forces.

At first, the police would not allow Shelton to visit the site, but they relented the next day after obtaining permission from YPG. There was not much to see. The victims, she learned, had been transferred to Avreen hospital in Afrin, a Kurdish-controlled town some 40 miles distant. The afflicted included members of the first response team, many of whom had to go to the hospital after developing symptoms of dizziness, headache, blurred vision or stomach pain. “There was a white powder covering the stairs, but there was no smell or smoke,” Shelton heard from Toul Haldun Zagroz, a Kurdish police officer with the second team on the scene who had also developed symptoms. He added:

When we went inside, we saw the children dead, and the Kurdish police who had arrived before us were on the ground foaming something white from their mouth. Their eyes were so red.

Later that day, Shelton went to an Internet café to call her editor, Gelling. “I pitched it as, this is a chemical attack. This is the big one. Let’s follow this,” says Shelton. “It was like, Pete, this is huge. I’m going to go check this out… The question was: was it one of those banned chemical weapons?” All the victims had suffered symptoms consistent with a chemical attack; few had visible physical injuries. Gelling instantly understood the importance, and agreed Shelton should pursue it. “The value from our perspective was very, very high, because if we can confirm or deny the existence of chemical weapons, that has major implications for the world,” says Gelling.

The truth is, whether we sent her there or not, she was probably going to go, because that’s who Tracey is… [Certainly] there was no means for us to get that story outside of Tracey. We weren’t going to work with freelancers we didn’t know. We were already tentative about things because of the Jim Foley situation. The only viable person to do that [story] would be Tracey, because we knew and trusted her.

On Thursday, April 18, Shelton set out for Afrin, a two-hour bus ride. Her friends went along because the wife was from Afrin. Shelton ended up spending five days in the city. First, she located Avreen Hospital Director Dr. Kawa Hassan. The hospital, he told her, had accepted 22 patients injured in Sheikh Maqsoud. They were unconscious or semi-conscious, foaming at the mouth and nose. He had equipped hospital staff with masks and protective clothing in case the chemical could be transmitted. The patients’ clothing and blankets were burned preventively. One—beyond those killed at the scene—died, but the others recovered within five days.

The most notable survivor was Yasser Younes, who remembered nothing but an explosion at 3 a.m. It was his wife and two children who had died. But Shelton could not interview him; Younes had returned to his home village. “He’d left to another village… and I was trying to get there, but I couldn’t get anyone to take me out there. I didn’t know the exact location” she recalls. Shelton had learned to be careful about accepting rides. In general, she felt safer with groups than individuals. “You had to be a little more selective about which groups you could trust to take you difference places, for transport through dangerous areas,” she says.

They’ve been known to capture Westerners… You had to be really careful that you had a secure ride, and you really trusted the people that you were with, and that they’d defend you… You just kind of collect people as you go along that you know you can trust with certain things.

Instead, she talked to people in Afrin who had talked to Younes, mostly hospital staff. Hospital Director Dr. Hassan was especially helpful “because he had some knowledge of the effects of these gases. He was very well prepared for it as well,” Shelton recalls. Dr. Hassan was persuaded that his patients’ symptoms were consistent with a sarin gas attack, a category of chemical weapon that would definitely trigger a US response. But he acknowledged that the attack had not had the devastating effects that full-on sarin would have had. He speculated that it was a weaker strain, had been watered down, or came from old stockpiles.

As for motive, Dr. Hassan theorized that the government wanted to scare the Kurds in Sheikh Maqsoud to prevent them working with the Free Syria Army, which the PKK had recently allowed into its area. Others thought perhaps the FSA had staged the attack to bring in foreign powers. Dr. Hassan had sent the canister recovered from Sheikh Maqsoud for testing to a laboratory in a Kurdish region, and was waiting for the results. Shelton waited as well, but the test results never came through. So on Wednesday, April 24, she returned to Aleppo to write her story.