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Beads of sweat drop from Mario Vergara's forehead as he, very carefully, kneels to examine the small fragment between the small rocks on the ground. To the untrained eye, it may seem like just another little white stone, but Vergara knows better.

“It's a bone fragment,” he said. A faint smile appears on his somewhat gaunt face, covered in the shadow of his sombrero. “Looks human.”

Some 10 minutes later, a forensic anthropologist of Mexico's federal Attorney General's Office (PGR) confirmed Vergara's discovery. The fragment is human, most likely from a rib. It is the last of some 10 pieces of human remains Vergara and some 30 volunteers found last week on the steep slope of a mountain near this small mining town in Mexico's southern state of Guerrero. The pieces were found in a line pattern, in what appears to be a shallow natural trench.

“Looks like this all belonged to the same body,” Vergara, 41, told Fox News Latino. He looked up toward the top of the mountain with a frown. “It was probably buried in a too shallow grave on top of the mountain and was then brought down by the rain.”

All in all, Vergara and his team of volunteers found some 20 bone fragments when FNL joined them on their search last week, including pieces of a skull, a pelvis and ribs. It was one of the most successful forensic runs the group has had since they first started looking for clandestine graves in the area in late 2014.

His group was the first of a growing number of citizen collectives that, all across the country, have begun searching for the estimated 27,000 to 40,000 people who have disappeared in Mexico since 2006.

In small groups, they scour the countryside hoping to find the remains of those being looked for, unfaltering, by their loved ones over the course of years. They are doing the work that the Mexican government, they say, is either unable or unwilling to do.

Comprised exclusively of civilians, these groups work with little resources and only the most basic of tools, like pick-axes and shovels.

The search party Fox News Latino joined in Mezcala scoured a hillside just outside town following the tip of a local resident, who said she had heard rumors of kidnapping victims brought to the area. She asked FNL to remain anonymous out of fear for her safety.

The group visited the site with a police escort and a forensic team of the PGR, who said they were not at liberty to comment. Vergara, who was part of the team, had no qualms in blasting the authorities.

“They went over this area several months ago, but they definitely didn't do a good job,” he said. “We still found bone fragments here. It's incredible that we're doing a better job than they are.”

The number of people disappeared is enormous, yet what happened to them often remains unclear. Kidnappings for ransom have been a staple of Mexican organized crime since the early 1990s, but don't account for the entire number of victims. Many have speculated that drug trafficking bands are kidnapping victims for slave labor, but those rumors often remain unconfirmed.

In Vergara's case, the tragedy hits close to home. His older brother Tomás, a taxi driver in the town of Huitzuco, not far from Iguala, has been missing since July 5, 2012.

“My town has endured kidnappings for years,” Vergara told FNL. “It's a business there for organized crime, and my brother became just another victim. Shortly after he disappeared, the kidnappers called us and asked for 300,000 pesos (approximately $15.000).”

He said that they were never given the proof of life they requested so did not hand over the money.

Vergara, who still fights tears each time he speaks of his missing brother, said he heard his voice twice in a recording. Then, after the fall of 2012 the kidnappers fell silent, and he never heard of his brother again. He and his sister Mayra now join the search effort every Sunday. Vergara has become the unofficial spokesperson of the group.

“Every time I find a bone fragment or some other human remain, it provokes both joy and sadness,” he said. “On the one hand I can hope that maybe it's my brother and we can finally get rest. On the other hand it's terrible to have to face so many deaths. We often say we are dead in life, because it's no way to live having to wait for a loved one who disappeared.”

Vergara's group calls itself “Los Otros Desaparecidos,” (The Other Disappeared). They are all family members of the many hundreds, perhaps thousands who were abducted in Guerrero in recent years as the state became Mexico's most violent and the battleground for a bloody turf war between drug trafficking gangs.

The Other Disappeared formed in Iguala, Guerrero's third biggest city, in the wake of the disappearance and probable murder in that city of 43 students of a rural teachers' college on September 26, 2014. The mass disappearance, according to the official investigation, was perpetrated by local crime group Guerreros Unidos, in collaboration with corrupt police officers and under Iguala's mayor orders – he is now in jail on suspected ties with organized crime.

The Iguala tragedy became one the defining moments of President Enrique Peña Nieto's administration, a tragic example of how politics and organized crime are all too often intertwined in Mexico, especially in traditionally lawless states such as Guerrero. The disappearance of the 43 students sparked widespread outrage on the country, especially after an independent group of experts criticized an apparently botched investigation into the mass disappearance.

National outrage notwithstanding, the tragedy also sparked something else: a sudden surge of courage in the minds of thousands of Mexicans who had loved ones go missing in Guerrero and other states suffering under Mexico's violent drug war.

After Iguala’s “The Other Disappeared” was created, other groups have followed suit and popped up in states like Coahuila and Sinaloa. Most recently, a new group has begun searching for the missing in the Gulf Coast state of Veracruz, another of the nation's most violent.

But setting up such a parallel justice apparatus in a country where the police is distrusted and accused of having ties with criminals is difficult and often fraught with danger.

The Iguala group was originally led by Miguel Ángel Jiménez Blanco, a member of the UPOEG, one of several community police organizations set up in rural Guerrero in mostly indigenous communities rife with organized crime and where local law enforcement has proved incapable of providing basic security.

Jiménez Blanco was gunned down in his hometown of Xaltianguis in August last year, allegedly by members of a criminal gang.

And in Veracruz, José Jesús Jiménez, one of the members of the recently created group called simply “Search Brigade,” was murdered just last month, on June 22. He was gunned down in the town of Poza Rica while searching for his daughter.

“The murder of José Jesús is a sign of how difficult it is for us to conduct these search efforts,” said Juan Carlos Trujillo, a human rights activist who has been involved in the organization of the brigades in Veracruz. “Some families obviously fear the danger [involved in] the search of their loved ones, but we need to continue.”

Mario Vergara agrees.

“We don't look for criminals,” he told FNL. “That would only place us in harm's way. When we run into a gang or something, we just avoid them,” he added. “We don't want any trouble.”