When filmmaker Ken Dornstein was 19 years old, his older brother David was one of 189 Americans killed in the bombing of Pan Am Flight 103 over Lockerbie, Scotland.

Nearly three decades later, only one suspect, a Libyan man, was ever convicted of the terror plot, which killed 270 people in total. Abdel Basset Ali al-Megrahi was sentenced to life in prison but later released; who else was involved remains an open question.

Who was really responsible for one of the worst terrorist attacks on Americans before 9/11?

In My Brother’s Bomber, an emotional and suspenseful three-part series that will open FRONTLINE’s new season on Sept. 29, Dornstein embarks on a quest for answers — diving deep into the case files, assembling a list of suspected plotters, and tracking them across the Middle East and Europe.

“If you talk to people who work with victims and survivors of crimes, having a satisfying account of what actually happened is really important,” says Dornstein. “I think there never was a satisfying account for me or for many of the other Lockerbie families.”

With each week’s episode, Dornstein, who previously wrote a book about his brother’s life — “The Boy Who Fell Out of the Sky: A True Story” (Random House, 2006) — encounters new witnesses and unearths fresh evidence that brings him closer to the truth about what really happened.

“I started this process with questions that were significant enough to get me to go over the Tunisian border into Libya,” he says. “I don’t have those questions any more.”

Unfolding in three one-hour installments on Tuesday, Sept. 29; Tuesday, Oct. 6; and Tuesday, Oct. 13, My Brother’s Bomber is a rare, real-life spy thriller, but also a timely reflection on the legacy of America’s long war on terror and a lyrical meditation on loss, love, revenge and the nature of obsession.

You can watch a sneak peek right now:

My Brother’s Bomber premieres Tuesday, Sept. 29 at 10/9c on PBS (check local listings).

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