When Ken Dornstein learned that Pan Am Flight 103 had exploded, he did not realize that his older brother, David, was on the plane. It was December 22, 1988, and Ken, a sophomore at Brown University, was at home, in Philadelphia, on winter break. Over breakfast, he read about the disaster in the Inquirer: all two hundred and fifty-nine passengers were killed, along with eleven residents of Lockerbie, Scotland, where flaming debris from the plane fell from the sky. David, who was twenty-five, had been living in Israel and was not scheduled to fly home until later that week, so Ken absorbed the details about the crash with the detached sympathy that one accords a stranger’s tragedy. That evening, the airline called. David had changed his plans in order to come home early and surprise his family.

Ken’s father, Perry, took the call. A successful physician, Perry was a stern and withdrawn parent; David had been boundlessly expressive, forever writing in a notebook or a journal. Their relationship had often been strained, and now the tensions between them could never be resolved. Ken felt that his father’s loss was “unspeakable,” and so they didn’t speak about it. Ken’s sister, Susan, told me that after the funeral Perry rarely mentioned David’s name again.

A hundred and eighty-nine of the victims were American, and, as news outlets across the country memorialized the dead, Ken felt that siblings “didn’t rank very highly” among surviving relatives. But he had adored David. Their parents had divorced when Ken was a toddler, and their mother, Judy, had struggled with mental illness and addiction. David had become protective of Ken, and had mentored him when he expressed an interest in writing. After the crash, Ken found a box among David’s possessions labelled “The Dave Archives”; it was stuffed with journals, stories, poetry, and plays. David had always seen himself as being on the verge of a celebrated literary career. Not long after his death, a local paper ran an obituary suggesting that he had written a novel in Israel. To Ken’s surprise, his father was quoted as saying, “He was about to submit the first part for publication.” This wasn’t true, and Ken was dismayed that his father had “rounded up” David’s literary achievements. (Perry Dornstein died in 2010, Judy in 2013.)

Ken arranged the journals chronologically and sorted the manuscripts into color-coded files. The process was eerie: David had sometimes suggested, mischievously, that he was destined to die young, and in the margins of his notebooks Ken discovered winking asides “for the biographers.”

When terrorists strike today, they often claim credit on social media. But Lockerbie, Dornstein told me recently, was a “murder mystery.” Flight 103 had left London for New York on December 21st, with David assigned to Row 40 of the economy section. After the plane ascended to thirty thousand feet, an electronic timer activated an explosive device hidden inside a Toshiba radio in the luggage hold, and a lump of Semtex detonated, shearing open the fuselage. The plane broke apart in midair, six miles above the earth. Many of the victims remained alive until the moment they hit the ground. But who built the bomb? Who placed it in the radio? Who put it on the plane?

For years, Dornstein said little to his friends or family about Lockerbie or about his brother. But he began applying the same quiet compulsiveness that he had channelled into the Dave Archives to the larger riddle of the bombing. He clipped articles, pored over archival footage, and sought out people who had known David. One day, at Penn Station in Manhattan, he spotted Kathryn Geismar, who had dated David for two years. They ended up on the same train, stayed in touch, and eventually fell in love. Initially, Ken hid the romance from his family, fearful that they might consider it an “unholy way to grieve.” But the relationship didn’t revolve around David; part of what comforted Ken about being with Geismar was that he didn’t need to talk with her about his loss. She already knew.

After college, Dornstein moved to Los Angeles and took a job at a detective agency. His colleagues knew nothing of his brother, but he privately took solace from accumulating investigative skills. “I was interested in the tradecraft of how you find people,” he recalled. He wondered about the shadowy culprits behind the Lockerbie bombing. “I wasn’t a worldly person, I hadn’t travelled,” he told me. “But I kept thinking, These guys are out there.”

When the F.B.I. dispatched agents to Scotland, it was the largest terrorism investigation in U.S. history. Debris from the plane had spread so widely that the crime scene spanned nearly nine hundred square miles. Initially, suspicion fell on a Palestinian terrorist group that operated out of Syria and was backed by Iran. But when Department of Justice prosecutors announced the results of the U.S. investigation, in November, 1991, they indicted two intelligence operatives from Libya. Prosecutors said that the Libyans had placed the bomb in a Samsonite suitcase and routed it, as unaccompanied baggage, on a plane that went from Malta to Frankfurt. It was then flown to London, where it was transferred onto Pan Am 103.

Throughout the eighties, Libya was a major state sponsor of terrorism. President Ronald Reagan referred to the Libyan leader Muammar Qaddafi as “the mad dog of the Middle East.” In 1986, after Libyan terrorists detonated a bomb at a Berlin disco that was popular with American soldiers, Reagan authorized air strikes on Tripoli and Benghazi. Qaddafi narrowly survived the bombing, which killed dozens, and some observers later speculated that Lockerbie was Qaddafi’s deadly riposte to this assassination attempt. But when the indictments were announced Qaddafi denied any Libyan involvement. He refused to turn over the two Libyan defendants until 1998, when he allowed them to stand trial at a special tribunal in the Netherlands. More than two hundred people appeared on the stand, but the testimony of one of the prosecutors’ key witnesses proved unreliable, and the prosecution’s case against the operatives was largely circumstantial. One of the suspects, Lamin Fhimah, was acquitted. The other, a bespectacled man named Abdelbaset al-Megrahi, was sentenced to life in prison. He was the only suspect to be convicted of the bombing.

Dornstein believed that Megrahi was guilty but had not acted alone. In 2003, Qaddafi released a carefully worded statement allowing that Libya might have been responsible for the blast, and he established a $2.7-billion fund to compensate the victims. But he never acknowledged authorizing Lockerbie. Brian Murtagh, the lead American prosecutor on the case, admitted to me that the plotters of the attack had eluded his grasp. “Our mandate was to try to indict everybody we could indict, not everybody we suspected,” he said. Dornstein recalls asking himself, “How could such a big act of mass murder have no author?”

Dornstein married Geismar, a psychologist, in 1997, and they settled in Somerville, Massachusetts. Ken began working for the PBS show “Frontline,” producing documentaries about Afghanistan and Iran. He developed a reputation as a tirelessly analytical researcher. All the while, he kept thinking about Pan Am 103. He travelled to Scotland and spent several weeks in Lockerbie interviewing investigators and walking through the pastures where the plane had gone down. He read the transcript of the Scottish Fatal Accident Inquiry, which exceeded fifteen thousand pages, and he located the patch of grass where David’s body had landed. He wrote about the trip in an article for this magazine, and in 2006 he published a book, “The Boy Who Fell Out of the Sky.” It is a tribute to David, drawing on his journals and other writings. “David left so many things behind, the beginnings of things,” Richard Suckle, a longtime friend of the family, told me. “In writing the book, it was as if Kenny had found a way that the two of them could collaborate.” The book also explores, with bracing self-awareness, Dornstein’s drive to investigate: “I had found a less painful way to miss my brother, by not missing him at all, just trying to document what happened to his body.”