At first glance, Ross McDonnell’s ambrotypes resemble 19th century portraits of desperados. Then you see the semi-automatic weapons and the ballcaps and realize McDonnell’s retro images depict men on the front lines of Mexico’s most fraught contemporary struggles.

An ambrotype is a photograph created on a glass plate that has been coated and sensitized in a chemical bath. Once exposed, the image is fixed, washed and developed. When the glass plate negative is turned around and placed against a black background, a positive image results.

McDonnell used the technique to great effect in Vigilantes, a series documenting Mexico’s Auto Defensa—bands of vigilantes who protect communities against the violence of the cartels. In some communities, they have all but replaced the ineffective policing of federal forces.

The movement spread westward from the state of Guerrero to Michoacán, two areas with large, and often poor, indigenous populations. The groups have enjoyed some success in those areas because the people there have little faith in the central government, says McDonnell. They are more likely to respect and trust those from within their community.

McDonnell started following the Auto Defensa in 2012 using the digital and color orthodoxies of photojournalism. He’s provided material for television news reporting, magazine publications and online video, using a variety of approaches and techniques.

“It has been great to work on a story like this and see how the meaning and ambience of the work can change depending on the context in which it is presented,” he says.

His decision to use ambrotypes grew from a collaboration with Galeria Grafika LaEstampa, a gallery in Mexico City that specializes in vintage photography. “I was initially a little bit skeptical about the process,” he says, “but the fact the plates are unique editions and their beautiful, ethereal nature of the results quickly convinced me that it would be a success.”

The Auto Defensa are a response to a drug war between the government and powerful cartels, a war that has seen more than 70,000 people killed. Rural areas like La Costa Chica in Guerrero, where McDonnell made his photographs, have suffered most. Auto Defensa police guard villages in these areas around the clock, arresting and detaining those suspected of crimes and trying them in popular courts.

Despite Auto Defensa groups’ entanglement in the potentially sticky political machinations of the region, McDonnell says they are largely effective community policing entities in Guerrero. It’s a different story in Michoacán, where the groups are at war Los Caballeros Templarios (the Knights of Templar), a quasi-religious cartel that specializes in systematic kidnapping and torture.

Seen against this backdrop, one might question McDonnell’s decision to use ambrotypes and argue he is aestheticizing violence and trauma. McDonnell doesn’t believe this to be the case; he points to Mexico’s history of revolution and upheaval, and how photographers have documented it from the earliest days of their medium.

“It’s interesting how those images were used and disseminated,” he says. “Photographs of Emiliano Zapata and images of the shirt he was shot in replete with hand written captions, postcards of dead and burned revolutionaries sent from the border as a way of relaying the news are common finds at Mexican flea markets.”

McDonnell also believes there is a powerful visual tension between the historic process and the vigilantes’ modern-day garb.

“An audience must ask themselves if these armed men appear to them to be good guys or bad guys,” he says. “In a way it says much about the level of crime and corruption in Mexico that armed, hooded vigilantes ostensibly represent the side of good in certain parts of the countryside, places where law and order have ceased to exist.”

“And, they are masked for their own safety.”