TORREON, MEXICO - Fifteen years after her daughter's disappearance, 55-year-old Silvia Ortiz spends day after day raking through arid scrubland in northern Mexico hoping to find her remains.

Ortiz belongs to a group of families working to uncover graves of some of the 40,000 people who have gone missing in mounting lawlessness since the government sent in the armed forces to tackle Mexico's drug cartels at the end of 2006.

More than 200,000 people have been killed in the violence since then, and shedding light on the fate of the disappeared is one of the toughest challenges facing the administration of President Andres Manuel Lopez Obrador.

Criminal gangs often bury the bodies of victims in clandestine graves. Some have been known to try to minimize evidence by dismembering or dousing them in acid.

Meanwhile, Mexican criminal investigators and forensic teams have struggled to keep up with the increased workload during the surge in killings. Fear of reprisals at the hands of perpetrators can also impede efforts to identify victims.

Decked out in surgical masks and sun hats, Ortiz's collective - Grupo Vida or "Life Group" - keeps up a near-constant digging operation around the city of Torreon in the state of Coahuila.

The volunteer association made up of relatives of local missing people set out six years ago to uncover clandestine graves, saying government officials were not trying hard enough to find or identify human remains.

"This is why we say we're facing a forensic emergency," said Ortiz, sitting at home with her husband, Oscar Sanchez, surrounded by framed photos of their daughter Silvia Stephanie Sanchez before she disappeared in 2004, aged 16.

She has not been heard from since.

Jose Angel Herrera, Coahuila's special prosecutor for disappeared people, said authorities are cooperating with Grupo Vida in the area around Torreon and conducting their own missions in other parts of the state.

In Coahuila, 2,144 people reported missing have never been found, Herrera said. Many disappeared while the territory was ravaged by the Zetas, a notoriously brutal drug cartel.

The state has identified the remains of 235 missing people through digs, including those led by Grupo Vida. -Reuters