SNEAK PEEK: This week Lens is featuring a selection of photographers attending the New York Portfolio Review.

The view from the hills around Iguala, Mexico, was stunning. But the more Christopher Gregory walked along the paths, the more his eye was drawn to the objects scattered along the way: scraps of clothing, beer bottles, trash. To him, these castoff items were possibly linked to the hundreds of people reported missing — presumably kidnapped, if not killed — by drug cartels that have long operated with impunity.

“I was attracted to the landscape and the idea these places and artifacts could tell us something about the missing,” he said. “This could have been the last landscape they saw. The beauty of the area is juxtaposed to what is happening in Mexico.”

Little more than six months after 43 students were abducted and presumably killed in Iguala in Guerrero State, Mr. Gregory is wondering about all the other people who have vanished in that region. He had wanted to do a project on the missing students, but abruptly changed his mind when, during the early stages of the search, a mass grave was found with the remains of 28 people.

None of them were the students.

That became a flash point for him and Jeremy Relph, a writer with whom he had teamed up for the story. Once they got to Iguala, they discovered that disappearances had been going on for years, and on an alarming scale. While the government has put the tally of missing people in Guerrero State at about 120 from January to November of last year, local advocates working with families reported that some 400 people had been reported missing in Iguala alone in recent years.

Mr. Gregory worked from a church where people filed reports of the missing and provided DNA samples. Working with Miguel Jimenez, who belonged to a local self-defense group, he met relatives and went out with them as they searched for graves, sometimes following up on reports of flickering lights seen at night in the hills.

While they found evidence of possible camps used by kidnappers, they did not find new graves. Still, it was difficult to work in the region, as he did not know if the cartel was still in the hills. And the cartel — Guerreros Unidos — had a fearsome reputation for extortion, kidnapping and murder.

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“There is a lot of mythology around the cartel,” Mr. Gregory said. “One of the biggest challenges for me was sussing out the truth, because there is no real way to verify these claims.”

One man he met there told how his 18-year-old son had disappeared not long after the students did. The father thought the boy had been dragooned to go to work processing and packaging drugs, because the area was close to poppy fields. “Maybe that is founded in truth,” Mr. Gregory said. “But it’s an attractive theory because it means your loved one is still alive.”

The fact that he vanished at a time when the area was presumably under close scrutiny by the news media and authorities is unsettling, however. And while people felt the need to file reports at the church, they also knew they were being watched.

“The intimacy of this place makes for a tense and volatile situation,” Mr. Gregory said. “For a lot of these people, their biggest fear after this ordeal was, how do they tread this delicate quasi-safe period they enjoy now. After the media and the military leave, what comes in? Is it a community police force? A new municipal police? I have never seen anybody as afraid of their local police as they were.”

Mr. Gregory intends to return to Mexico and continue his project, which he called “Evidencia.” To him, his images speak to proof of things that have gone unseen, but not unfelt by almost everyone in Iguala.

“The photo is an evidentiary document,” he said. “There is no way to witness these kidnappings or document these violations of human rights, other than to point at the residue and try to have a conversation about what it means, how it looks like and how do we navigate these complex social and political issues, as well as the psychological issues. You can’t believe anybody or trust anybody in these areas because for all intents and purposes, they’re lawless.”

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