Maria Herrera is scraping at the earth on a hill in the town of Huitzuco, in southern Mexico, looking for the mounds or sunken spots that indicate a decaying corpse.

At 70 years old, Herrera is hoping against all odds to find her four missing sons – two who disappeared in 2008, and two who vanished in 2010 looking for their brothers.

Forensic personnel work to exhume human remains found during the fourth National Search Brigade, in Huitzuco de los Figueroa

“Every time we come to one of these nasty places, we suffer … Who heard their screams of pain? Who heard their last words?” she said through tears as she dug in the dirt with a group of 100 other activists in the violent state of Guerrero.

The small, grey-haired grandmother is the face of a dirty secret that has haunted Mexico for years: the countryside of Latin America’s second-largest economy is littered with bodies.

Maria Herrera, who is lookings for four missing sons, in Huitzuco de los Figueroa.

More than 40,000 people are missing in Mexico, which has been swept by a wave of violence since the government declared war on the country’s powerful drug cartels in 2006.

Herrera regularly goes out searching for her sons with other relatives of the “disappeared”.

But she is also part of a smaller, even more tragic group: some 20 families who have lost children not once but twice, when the ones who remained went looking for their missing siblings and ended up disappearing too.

Herrera’s family comes from Pajacuarán, a small town in western Mexico where most people are farmers or emigrate to the United States.

A mass held as part of the activities of the fourth National Search Brigade, in Huitzuco de los Figueroa

She and her husband decided they wanted something different for their eight children. They started a small business selling household goods door to door, then used the profits to launch a nationwide gold exchange.

Part of the business, she said, involved traveling the country to buy and sell gold – which is what Jesus Salvador, then 24, and Raul, then 19, were doing in Guerrero in 2008.

Traveling with five employees in an SUV carrying nearly $90,000 (£70,000) in cash and gold, they did not realise a bloody cartel turf war was just breaking out in the state.

A police dog is used during the fourth National Search Brigade

“My brothers had no idea when they arrived,” said Juan Carlos, 41, their older sibling.

He and his family believe a local cartel mistook the brothers and their co-workers for members of a rival group and had some crooked cops arrest them.

Such stories are not uncommon in Guerrero. It is the state where 43 student protesters disappeared in 2014 after being arrested by state police, who apparently handed them over to cartel hitmen – a notorious case that drew international condemnation, and remains unsolved.

A boy (right) assists the Search Brigade (top). Left: Mario Vergara, who is looking for his brother, displays a banner

With no news of their sons, and fed up with the lack of answers from the authorities, the Herreras hired private investigators and began searching on their own.

Their situation got more desperate in February 2009, when Herrera’s husband died of a stroke.

Taking up the family gold business – and using their travels to search for Jesus and Raul – two more brothers, Gustavo, then 27, and Luis Armando, then 25, started criss-crossing the country.

Children dress in white for a march in Huitzuco de los Figueroa organised by the Search Brigade

“Sometimes the drug traffickers take their victims somewhere else. Maybe we’ll find them in another state,” Herrera said Gustavo told her.

They were on such a trip when they, too, disappeared.

Moments after Gustavo called his wife to check in, on 22 September 2010, the brothers were detained by police in Poza Rica, in the eastern state of Veracruz – another cartel hotspot known for hit squads run by corrupt cops.

More than 40,000 people are missing in Mexico, which has been swept by a wave of violence since declaring war on its powerful drug cartels in 2006

The family believes the police decided to get rid of the pair when they realised they were searching for missing persons.

Looking for the missing can be dangerous in Mexico.

Herrera’s latest group, the Fourth National Missing Persons Search Brigade, had to be escorted by federal police.

Juan Carlos, her 41-year-old son, was attacked by an unknown gunman six months ago while organising another search party. He managed to escape by jumping over a wall.

Members of the Brigade concentrate their search

Herrera joined her first search party in 2016, in Veracruz, and has since become an expert, learning the trade from forensic anthropologists – things like how to hammer metal rods into the ground in a T to release any smells of decaying flesh.

Her latest group found seven bodies during its two-week search. Others like it have found many more.

Officials say there are probably more than 1,000 unmarked burial sites in Mexico.

“Unfortunately, the country has become a giant clandestine grave,” Mexico’s under-secretary for human rights, Alejandro Encinas, said recently.

Forensic personnel of the Mexican attorney general work in the exhumation of human remains found by the fourth National Search Brigade in Huitzuco de los Figueroa

On top of that, there are about 26,000 unidentified bodies in the forensic system, according to the government of the Mexican president, Andrés Manuel López Obrador, who took office in December.

His government recently announced a new plan to search for the missing, including a new forensic institute.

Identifying the bodies languishing in the system would be a good start, said Herrera.

“We’ll keep looking. But please, for the love of God, let them identify the ones we’ve already found,” she said.