WATCH: The List That Might Unlock the Lockerbie Bombing Mystery

If one of your family members were killed in a terrorist attack, and years later, you still had questions about who was responsible, what would you be willing to do to find the truth?

For author and filmmaker Ken Dornstein, whose older brother, David, was killed in the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, the answer involved conducting his own investigation — one that picked up where the official inquiry left off.

“We got part of the conspiracy, but only a small part,” Stuart Henderson, the retired lead Scottish investigator on the Lockerbie case, tells Dornstein in My Brother’s Bomber, a three-part FRONTLINE documentary about Dornstein’s search that begins tonight on PBS.

That “small part” was a Libyan man named Abdel Basset al Megrahi, the only person ever convicted for the bombing. Megrahi was sentenced to life in prison, but in 2009, dying of cancer, he was granted a compassionate release and sent home to Libya under a cloud of controversy and suspicion.

Was Megrahi — who claimed innocence until his death in 2012 — who he said he was? And who else might have been involved in planning one of the worst terror attacks on Americans before 9/11?

For Dornstein, the only way to know was to amass a list of names, travel to Libya and ask them in person — something investigators told Dornstein they could never do while Libyan leader Muammar Qaddafi was still in power.

So when Qaddafi was ousted and killed in 2011, Dornstein saw a unique opportunity to come face-to-face with other potential suspects in the attack that killed his brother and 269 others.

And he had an idea of where to start.

“Stuart Henderson and I, we both left lists with our successors to say, if you get to Libya, this is what you ought to do. This is who you ought to go after,” Richard Marquise, the retired FBI agent who helped lead the international investigation of the attack, tells Dornstein in the below excerpt from the film.

“These are the people that must be found,” Henderson says, handing Dornstein his list of suspects. “I never ever got access to them long enough to interview any of them.”

Who were the people on the list? If confronted about Lockerbie, what would they say?

The investigation would mean leaving his home and family behind for war-torn Libya, but Dornstein couldn’t pass up the chance.

My Brother’s Bomber begins Tues., Sept. 29 on PBS; Parts Two and Three will air Tues., Oct. 6 and Tues., Oct. 13. Check your local PBS listings here.