Carol King-Eckersley, 65, from Oregon, decided last year to search for the baby boy she gave up at birth when she was 19, according to a BBC documentary.
She knew his adopted name -- Kenneth Bissett -- but her hopes of a reunion were dashed when earlier this year she found the name on a Lockerbie remembrance page.
"270 people died in that tragedy and one of those happened to be the only child I ever had. And I didn't even know it until last April," King-Eckersley told the BBC.
Pan Am flight 103 from London to New York blew up on December 21, 1988, killing all 259 people on board and 11 more on the ground in the Scottish town of Lockerbie.
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King-Eckersley told the BBC that she had for decades kept a promise not to look for her son after giving him up for adoption in 1967.
But she began the search as part of grief counselling after her husband died in 2012.
His birth mother found his name on a website for students who had died in the bombing and at first could not believe that it was him.
"And it finally dawned on me that it was right, and I just said 'my God, my baby's dead," King-Eckersley said.
She added: "There was always the hope and dream that some day there would come a knock at the door and I would open it and there would be this tall handsome gentleman saying, 'Hi, I guess you're my mom'.
King-Eckersley attended a remembrance service at Syracuse University in New York state, which lost 35 students in the Lockerbie bombing and had arranged the exchange programme Kenneth Bisset was on.
Bissett's adoptive parents are both dead.