Forensic experts in Colombia have begun a search for dozens of bodies at a landfill site believed to be one of the largest urban mass graves in the world.
Relatives of possible victims held a ceremony at the site on the outskirts of the city of Medellin before the excavation started.
The bodies of 90 to 300 people are thought to be buried there.
The disappearances date from 2002, when the army launched an operation against left-wing rebels in the area.
The operation was ordered by Colombia's president at the time, Alvaro Uribe.
Right-wing paramilitaries filled the void when the rebels left the Comuna 13 shantytown area and they are blamed by many for most of the killings.
Criminal gangs are also accused of involvement in some of the disappearances.
Medellin was once considered one of the world's most violent cities.
It was the home of the Medellin Cartel, the drug-trafficking organisation led by Pablo Escobar, who was killed in 1993.
Some 20,000 tonnes of earth will be removed over the next five months in the search for the bodies, reports the BBC's Natalio Cosoy in Bogota.
'Drop of hope'
A ceremony at the site, including a religious service, marked the beginning of the excavation.
"It took us 13 years to get here. This is a drop of hope," said Luz Elena Galeano, leader of an organisation of women fighting for justice for their missing relatives.
Relatives laid flowers and images of their loved ones on the site.
"The ceremony was moving and a commitment to peace and reconciliation," said Colombia's Interior Minister Juan Fernando Cristo.
More than 200,000 people have been killed since hostilities between the army and Colombia's main rebel group, the Farc, began in 1964.
Both sides have been engaged in nearly three years of peace negotiations, which are being held in Cuba.
Earlier this month, the Colombian government announced a de-escalation of attacks against the rebels, who had announced a unilateral ceasefire.
The talks are aimed at ending hostilities, which would lead to the Farc giving up its armed struggle to join the legal political process.
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Margarita Selene Restrepo stares out over the corrugated roofs of Comuna 13 - one of Medellin's poorest and most violent districts. From here, a few steps from her home, she can see a huge, deforested, earthen scar on the hillside opposite. In Spanish it is known as la escombrera - the dump. And Margarita can just make out areas recently fenced off with flimsy green plastic.
"Every day when I look across there it causes me such a lot of sadness. If she's there, she's so close. Yet at the same time, she's so far away."