The last contact Margarita Restrepo had with her daughter was a hurried phone call on Oct. 25, 2002. The school day was over and 17-year-old Carol Vanesa was going to meet friends at a metro stop near the sprawling Comuna 13 hillside slum.

Restrepo and her children had fled the violent Medellin neighborhood a few days earlier, right before it was taken over by thousands of Colombian soldiers trying to ferret out leftist rebels. She begged the girl not to risk returning there, but the teen went anyway. Neither she nor her two friends have been seen again and, to this day, nobody knows who is responsible for their disappearance.

Thirteen years later, Restrepo and dozens of others who have missing loved ones are closer than ever to closure. On Monday, a team of forensic experts will begin removing 31,000 cubic yards (24,000 cubic meters) of rubble from La Escombrera, a debris landfill on Medellin's outskirts where the remains of as many as 300 people are believed to have been dumped during one of the darkest chapters of Colombia's long-running civil conflict.

Human rights activists say it could be the biggest mass grave ever in Colombia and the dig represents a glimmer of hope that justice will be realized. But the search will be complicated. Despite more than a decade-long clamor by victims' families that the landfill be closed and excavated, giant trucks have continued to dump construction waste daily.

"If that light doesn't shine for me, I hope it does for one of my companions," Restrepo said while holding up a placard with her daughter's photo and disappearance date, the eye-catching symbol used by the group Mothers Walking for the Truth to draw attention to their fight.

Colombia's rightist paramilitary groups demobilized a decade ago, and the government is now negotiating a peace deal with the biggest rebel movement. With the five-decade conflict winding down, officials have been fanning out across the country to exhume hundreds of bodies, attempt to identify them through DNA testing and return the remains to family members.

But most of the unmarked graves are located in lawless rural areas, not Medellin, which is Colombia's second-largest city.

Restrepo's disappearance took place at a time and place where being young like her was almost tantamount to a death sentence.

Shortly after taking office in 2002, then President Alvaro Uribe launched Operation Orion to repel leftist rebels from a densely populated hillside slum in the poor and violent Comuna 13 district. The offensive lionized Uribe's reputation among Colombians as a crime-fighting conservative whose tough talk was backed by action.

But almost as soon as the military retrenched, the void was filled by far-right militia fighters in ski masks and wielding heavy weapons. Allegations of killings of civilians and disappearances multiplied. Many of the paramilitary crimes were carried out in an alliance with U.S.-trained security forces.

Former militia fighters, including Diego Fernando Murillo, the jailed warlord known by the alias Don Berna who once terrorized much of Medellin, have testified they dumped their victims in La Escombrera.

Investigators say it is unclear how many, if any, bodies can be recovered. Too much time has passed and the 8-meter-high (9-yard-high) mountain of debris likely has crushed many of the remains.

But they say their biggest obstacle is providing for the safety of the forensic experts carrying out the painstaking work. While violence in Medellin's slums has fallen sharply over the past decade, with the city last year reporting its lowest homicide rate since the height of drug boss Pablo Escobar's power in the mid-1980s, the five-month excavation is taking place in an area where criminal gangs still lurk, many of whose members are implicated in the very crimes being investigated. Mistrust of the police, who are providing around-the-clock protection, still runs high.

Still, the experts will try to help close a wound not just for the victims of Comuna 13 but to make a symbolic gesture for millions of Colombians touched by violence and abandoned by the state.

They have cordoned off and will focus their search in three sections of the landfill where bodies are believed to have been dumped. A makeshift camp is being built for loved ones who want to stay abreast of the investigators' progress. There is also a mausoleum planned and an exhibit in a Medellin's Museum of Memory, a new space for reflection and study of Colombia's violent past.

"For years we denounced forced disappearances on a large scale and nothing was done," said Adriana Arboleda, a lawyer working with victims to sue the state at the Inter-American Court of Human Rights. "Imagine how much money and effort could have been saved if they had listened to us earlier."

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Joshua Goodman on Twitter: https://twitter.com/apjoshgoodman