Oscar had reunited with his teenage sweetheart, Nidia, left, in 2005, and worked two full-time jobs to support his growing family, including — most recently — their 9-month-old daughter Dulce, seen in Oscar’s arms on a recent Sunday in May. (Matthew Healey for ProPublica)

When Oscar got homesick, he often took out an aging photo album that belonged to his father, Lt. Oscar Ovidio Ramírez Ramos, who had died when Oscar was four. (Matthew Healey for ProPublica)

After Lt. Ramírez’s death, Oscar, seen in the front at age 8, was raised by the lieutenant’s mother and sisters in Zacapa, Guatemala.

The Ramírez family taught Oscar, seen here at age 15, to revere the lieutenant, a member of the Guatemalan Army special forces called Kaibiles.

Lt. Ramírez had graduated at the top of his class from military school, become an elite commando and won medals in combat. He was deputy commander of the assault squad that attacked the village of Dos Erres in 1982. Ramírez died in a truck accident less than a year after the assault. (Matthew Healey for ProPublica)

Lt. Ramírez was admired on the battlefield for his prowess and loyalty, according to some who had served with him. He was a dutiful son, who wired money to his mother each month. (Matthew Healey for ProPublica)

The lieutenant's nickname was Cocorico, a diminutive of Oscar, who called himself Cocorico the Second. (Matthew Healey for ProPublica)

Sara Romero is a special human rights prosecutor in Guatemala who investigated the assualt on Dos Erres, which left 250 civilians dead. In May 2011, she emailed Oscar to tell him that she had reached an inescapable conclusion: Oscar was one of the boys who had survived the Dos Erres massacre and had been abducted by the commandos. (Habiba Nosheen)

Oscar struggled to reconcile his memories with the prosecutor’s words. If her theory was correct, Oscar had been kidnapped by the man he admired. Romero offered to arrange a DNA test to test her theory. Oscar agreed. (Matthew Healey for ProPublica)

It took about six weeks for the DNA results. When Oscar, seen here with Nidia, left, and Dulce in his arms, finally heard the results, they came with a surprise: Oscar’s biological father was still alive. (Matthew Healey for ProPublica)

Tranquilino Castañeda, a farmer, was away visiting relatives on the day of the Dos Erres massacre. For nearly 30 years, he believed that his pregnant wife and all nine of his children had been murdered. (Alex Cruz/El Periódico de Guatemala)

Once the DNA results proved that Tranquilino and Oscar were father and son, human rights investigators set up an emotional video conversation reuniting the two. (Alex Cruz/El Periódico de Guatemala)

Soon, the father and son were speaking every day and filling in three missing decades. Oscar labeled the folder on his computer with photos of Tranquilino as "mi padre," my father. (Matthew Healey for ProPublica)

Oscar hasn't forgotten the lieutenant. While he understands that Lt. Ramírez participated in the violence at Dos Erres, Oscar, seen here looking at the aging photo album with his daughter Nicole, 7, holds on to two facts: The lieutenant saved him. And the Ramírez family treated him as one of their own. (Matthew Healey for ProPublica)

Oscar is in the United States illegally. His petition for political asylum is pending. Investigators believe he can help convict those responsible for atrocities at Dos Erres. "I imagine there is a connection between the violence of the past and the present," Oscar says. "If you don't catch these people, it keeps spreading. People do whatever they want." (Matthew Healey for ProPublica)

Finding Oscar has given Tranquilino, seen holding a photograph of his son and his family, a new mission in life. He is scheduled to meet Oscar in person in the United States. (Alex Cruz/El Periódico de Guatemala)