The truth is an elusive, much disputed, and highly valuable commodity in Reynosa, Tamaulipas, a sprawling border city fifteen minutes south of McAllen. Residents witness a shootout that leaves dozens dead, and the government reports a minor disturbance. A businessman receives a call from “kidnappers” demanding immediate ransom, then discovers there is no actual kidnapping. Fireworks are mistaken for grenades. Grenades are mistaken for fireworks. The bloody conflict over turf and power that has taken the lives of tens of thousands of people isn’t the only war going on in Mexico. There is a second conflict over the story of what is happening—a clash that involves far fewer bullets but is no less real.

On a night early in December, Reynosa appears, at least for the moment, to be bustling but at peace. The main thoroughfare, Boulevard Hidalgo, is packed. Men lounge at roadside taquerias. After-work exercisers sweat through a Zumba class. “This used to be a ghost town,” Sergio Chapa, a Harlingen TV reporter, tells me as we zoom through the city in the back of a cab. After the Gulf cartel and its former enforcement wing, Los Zetas, went to war in 2010, Boulevard Hidalgo would often lie empty at night. Now life is returning to a semblance of normalcy. “It’s good to see it with traffic,” he says, staring out the window.

This particular night is uneventful, but Reynosans know better than to trust the calm and know much better than to trust stories about it. The Reynosa and Tamaulipas governments have an interest in understating the violence, and the Reynosa press essentially stopped reporting on the cartels years ago out of fear. (Reporters Without Borders ranks Mexico between the Democratic Republic of Congo and Iraq on its World Press Freedom Index.) It has become common practice for organized crime to infiltrate Mexican newsrooms and instruct journalists on what they can and cannot write.

This black hole of credible information has led to the emergence of new voices. Over the past five years, one of Reynosa’s most trusted news sources has been the man whom Chapa and I have come to meet: an anonymous Twitter user known as Chuy.

Chuy, who tweets under the handle @MrCruzStar, meets us at a mall a few miles up Boulevard Hidalgo, and the three of us make our way by taxi to his house. In the cab, it’s all small talk. His Twitter activities, after all, are secret. But once we arrive safely at his home, we discuss how he helps coordinate a network of three thousand or so Twitter users who report disturbances throughout the city using the hashtag #ReynosaFollow. On any given day or night, #ReynosaFollow collects dozens of posts warning of a shootout or a blockade or a column of armored vehicles. It’s essentially a 24-hour neighborhood watch for a city of nearly one million people, enabling citizens to know where they can—and can’t—travel safely. “If we didn’t have that information, the fear would make you stay at home,” Chuy says.

But just two months before, early on the morning of October 16, #ReynosaFollow became a vehicle for spreading fear rather than assuaging it. At 3:04 a.m., a tweet was posted from the account of a much-followed user known as Felina. “Friends and family, my name is María del Rosario Fuentes Rubio, I am a doctor, today my life has come to an end,” it read, in Spanish. Two more tweets arrived over the next five minutes: “I have nothing else to say but do not make the same mistake as I did. You do not win anything. To the contrary I now realize that I found death in exchange for nothing. They are closer than you think.” The final tweet came at 3:11 a.m.: “Close your accounts, do not risk your families as I did with mine. I ask for forgiveness.” Embedded in that tweet were two photographs, one of a woman, presumably Fuentes, staring impassively into a camera, another of the same woman faceup on the ground, blood trickling from her nose, apparently executed.

In a matter of hours, Chuy noticed that accounts were disappearing by the dozen. “We lost reliable sources who self-censored out of fear,” he says. “Now, if something happens, we won’t have the same panorama we had before. We’ll be missing those eyes.”

The first tweet bearing the #ReynosaFollow hashtag appeared on February 23, 2010, at 1:15 p.m. Before the night was out, hundreds of tweets had appeared. A month earlier the Gulf cartel–Zetas split had taken place, and violent clashes were now routine throughout the city. At first, Chuy says, the authorities “tried to paint us as paranoid liars.” But the violence was so bad and the evidence on Twitter so irrefutable that soon the Tamaulipas and Reynosa governments began to issue warnings about “situations of risk” on their official online accounts.

The pioneering #ReynosaFollow users weren’t trained journalists, but they adopted a set of best practices that would fit comfortably in any newsroom. Wary of spreading rumors and misinformation, they privileged primary-source reports—“I’m seeing,” “I’m hearing,” “My mom called and told me.” But Chuy would treat even a purported eyewitness account skeptically; only after seeing three users describing the same event would he consider the information credible. Then he would try to confirm it himself before posting an update to his Twitter feed, which has more than 10,000 followers.

Early on, an inner circle of users forged a community, meeting at “tweet posadas,” essentially #ReynosaFollow barbecues. But social media is an open platform, and the drug war in Mexico has many actors with many agendas. As #ReynosaFollow grew, so did online currents of misinformation, deception, and sabotage. Anonymous accounts issued false reports via #ReynosaFollow, and bots—automatically generated accounts—published thousands of tweets bearing the hashtag, effectively burying citizen updates under an avalanche of erroneous information. Chuy also remembers seeing tweets from shadowy users who seemed to have “way too much information” about the inner workings of cartels. “They wanted us to become a mouthpiece for them,” he says.

In May of last year, after the government launched a new security strategy, which included a call for citizens to report anonymous tips on cartel members and criminal activities, another contingent made itself known. Chuy noticed a preponderance of new accounts repeating the same message: “Don’t be a coward. If you don’t help, this will never end,” and concluded that many of the accounts were, in fact, “sock puppets”: bogus citizen accounts that were created by law enforcement, military, or paramilitary interests. “The way they write, the phrases they use—it isn’t the local language or the language of the people,” Chuy tells me.

Sometimes the attacks were more personal. In December, after Chuy contributed to two articles on the website Diario19 documenting the possible paramilitary affiliation of the popular online “citizen journalist” group Valor Por Tamaulipas, a new account called Mr. Fashion Cruz (a riff on Chuy’s handle) started tweeting angrily at him. The account’s avatar was a photograph of the real-life Chuy, a chilling message that someone had identified the man behind @MrCruzStar. (Valor Por Tamaulipas launched its own attack in early January, decrying Chuy as secretly working with the state attorney general’s office and accusing him of exposing Felina’s identity before her death.)

Felina’s apparent murder occurred not only against this background of distrust but also at a time when the #ReynosaFollow community was tearing itself apart. In early 2014, according to a prominent user who goes by the Twitter name Don Alejo, a rift had developed over the direction of the group. One faction believed that the hashtag should restrict itself to informing citizens about dangerous activities. The other took a tone that aligned it closely with the state. “They wanted to denounce the criminals,” Don Alejo says, “and they got very aggressive.”

Don Alejo and Chuy were in the first camp. Felina was very much in the second. Don Alejo, who says he knew Felina personally, remembers her as “very active, very happy, very full of life.” But on Twitter her persona was ferocious. “She would say, ‘Que maten a esos perros’ (‘May they kill those dogs’).” Chuy saw Felina’s activism as crossing a perilous line. “She’d say, ‘This person does this and lives here.’ Her activities were a red flag.”

It is not clear exactly what happened to Felina. The dominant theory, first reported in the Mexican magazine Zócalo and advanced by Vice News, is that Dr. María del Rosario Fuentes Rubio, a 36-year-old general practitioner, was kidnapped after a child died in her care. Someone had wanted revenge—or at least answers—and had taken her as she left her hospital on the morning of October 15. While she was detained, Fuentes’s kidnappers—presumably affiliated with organized crime—sifted through the contents of her phone and discovered her Twitter activity. Then, says Don Alejo, “it was like killing two birds with one stone.”

Others have their doubts. Chuy’s collaborator on the Valor Por Tamaulipas articles, UT-Brownsville professor Guadalupe Correa-Cabrera, says she had come to believe that Felina had paramilitary ties because of the specificity of her attacks on organized crime members and her close association with Valor Por Tamaulipas, to whose website she had contributed.

The lack of any confirmed account of Fuentes’s death has even made Correa-Cabrera wonder whether she was really murdered. No body has been found, no details of the investigation have been released, and no criminal organization has claimed responsibility. “I would not say that [Fuentes] is not dead or that the assassination and kidnapping were not committed—that would be very irresponsible on my part,” she says. “But I have big doubts. This person who was always behind a computer—all we know about her is from other anonymous social media users. This fills me with questions, not with answers.”

Outside Chuy’s house, a festival is taking place to celebrate the Feast of the Virgin of Guadalupe. As fireworks pop around us, I ask Chuy what he makes of Correa-Cabrera’s suspicions. He says he too had noted some irregularities. For the first several years that Felina was on Twitter, Chuy regarded her activity as nothing out of the ordinary. But in late spring 2014, he observed a change. She began to post photographs of crime scenes that looked like they had been taken by someone inside the law enforcement perimeter, which led him to believe that she was now collaborating with military forces. “It was notable that she began a very active campaign of denunciations at the same time as the new security strategy for Tamaulipas was launched,” he says. But he doesn’t doubt that she was murdered. “Here in Reynosa, if you come up on a narco blockade, the first thing they do is check your phone—your photos, your contacts, your messages—so they probably found her that way,” he says.

Whatever the case, the grisly tweets of Felina’s apparent death have had the desired effect. Even Chuy says he has decreased his activity; he is now even more keenly aware of how each tweet offers a clue to anyone who wants to find him. Still, he tells me, this terrible event has had a silver lining. Some of the feuding members of #ReynosaFollow have been brought back together. “I tell two people when I’m going to meet someone at X place, and we use an app that lets someone monitor where we are,” he says. “We keep an eye on each other.”

After leaving his house, Chuy, Chapa, and I wend our way through the narrow streets and come upon rows of dancing children. They are twirling to a pulsing drumbeat while their families look on, taking pictures and drinking beer. Reynosa is still violent, but people are getting on with their lives. #ReynosaFollow, however imperfect, has helped them maintain normalcy.

Once Chapa and I hop into a cab to head back to the bridge that connects downtown Reynosa with Texas, our driver peppers us with questions about what we were doing in Mexico. During a lull in the conversation, he turns on his stereo, and the vehicle is soon vibrating with a strutting bass line. I look up and see the word “Sicario” (“Hitman”) flashing across the car’s digital display. The song is a narco hip-hop track, a triumphalist anthem boasting of the exploits of the Gulf cartel and their fearsome assassins.

“Are you afraid?” I had asked Chuy back at his house. “What would happen if the wrong people knew where you lived?”

He paused. “They always ask me if I’m afraid because of what I do, and I say I’m scared as a citizen. I’ve been robbed. Sometimes people die from stray bullets in a shootout. I think everyone is at risk in this city.”