Katrina Browne was studying at seminary when her grandmother sent her a booklet she'd written on their family history. As she read it, one sentence stunned her: It mentioned the DeWolfs' slave-trading business out of Bristol, R.I.

"The first shock was as if finding out for the first time the horror of being descended from slave traders," she says. "But within moments, I realized I already knew, and yet had completely buried it."

It was that second shock of recognizing her own "amnesia" that spurred Ms. Browne to dig into the history further, and the surprises continued. The DeWolfs, she learned, created a wealthy dynasty that became the largest slave-trading family in early America. She assumed those forebears were an exception, but found they were part of a broad pattern of Northern participation in slavery.

To explore what that participation meant for her family and the country, Browne contacted all the relatives she could identify, inviting them to travel their ancestors' trade route from Bristol to Africa and the Caribbean. Nine other DeWolf descendants signed on, and last week, a documentary of their journey produced by Browne premièred at the Sundance Film Festival. "Traces of the Trade: A Story from the Deep North" was purchased by PBS and will be shown as part of its Point of View (P.O.V.) series.