Nelson de Witt had just returned from summer camp when he heard the life-changing news.

It wasn’t the fact that Mr. de Witt’s biological family was located in El Salvador – he had long known he was adopted from the Central American nation.

But for the first time the details of his life before adoption were revealed – and the story was more dramatic than he ever imagined. Then 16-year-old de Witt learned he wasn’t just adopted at age two by a family in Boston; He was the son of El Salvadoran revolutionaries.

Born Roberto Coto, his mother died in a government raid on a guerrilla safe house in the Honduran capital of Tegucigalpa and de Witt became one of the estimated 800 disappeared children of El Salvador’s 12-year civil war, which ended in 1992 with a peace accord between the government and leftist guerrillas.

In the years following his discovery – which was facilitated by an emotional letter from his biological grandmother ­– de Witt was able to meet the surviving members of his birth family, which included one brother and two sisters, the grandmother, and his father. It was a journey that took him across Central America, from El Salvador to Costa Rica to Panama.

De Witt was lucky, he learned.

More than 75,000 people died in El Salvador’s civil war, and rights groups have uncovered that it was common practice for soldiers to "disappear" children – in many cases either taking them and raising them as their own, or selling them through illegal adoption networks.

In a memoir titled Missing Mila, Finding Family: An International Adoption in the Shadow of the Salvadoran Civil War, de Witt's adoptive mother, Margaret E. Ward, references the shepherding role played by the wife of then US ambassador to Honduras John Negroponte in her son's adoption.

"As we considered the little we had been told about our adopted son and how he had been orphaned, it did occur to us that the background might be highly political," Ms. Ward wrote.

De Witt set out in his own small way to try to help others like himself and families who had lost young children during the civil war. And his summer camp in East Brookfield, Mass. again played a central role: De Witt, now in his early thirties, teamed up with his first camp counselor, John Younger, a professional filmmaker, to produce a documentary about his life story and El Salvador's myriad disappeared children. The project has been funded through Kickstarter, an online fundraising tool.

Titled "Identifying Nelson," the film follows de Witt on his 15-year journey toward understanding his past and his birth-country's history. He builds relationships with members of his biological family, meets the El Salvadoran president Mauricio Funes, and interviews other “disappeared” children.

In one of the opening sequences, a grainy newspaper photo appears on the screen, taken after Honduran authorities conducted the raid that killed de Witt's mother. In the arms of a woman wearing military fatigues, a two-year-old de Witt stares down the lens. "I am Nelson de Witt. I am Roberto Coto," de Witt narrates. "I am one of the disappeared children of El Salvador."

As the film progresses, we learn of his birth family's 14-year search to find him – particularly the tireless efforts of his grandmother. That long struggle is perhaps encapsulated best in a handwritten letter she penned after learning there was a chance her grandson might have been located in the US. Her words flash across the screen, a particularly poignant sentence highlighted. "You can't imagine how long I've been looking for you," she wrote. "Every night I prayed and asked God to help me find you..."

In another scene, we see newspaper photographs of a woman's bloodied corpse slumped on the floor. "As part of an operation for the FMLN [El Salvador’s rebel group during the civil war], my mother was involved in a kidnapping for ransom of a businessman, which ultimately led to my disappearance," de Witt says. "We don't know exactly what happened but we are sure my mother did not make it out alive."

The film also documents de Witt’s reunion with his biological family: A photo of him greeting his father for the first time trails across the screen, their likeness striking.

"Identifying Nelson" documents de Witt’s personal voyage, but he envisions the film as a way to highlight the issue of El Salvador’s disappeared and to pressure authorities to continue uncovering the truth of what happened to them. “Thirty years on and families still are looking for loved ones,” he says.

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The film is a three-part series, and Mr. Younger and de Witt have put on screenings of part one, due out in early 2014, at university campuses across the US.

For updates on the film’s release and progress, visit www.identifyingnelson.com.