In “Say Nothing: A True Story of Murder and Memory in Northern Ireland,” Patrick Radden Keefe trains a cold eye on an incendiary subject. Despite having Irish ancestors who immigrated to the United States in the 19th century, Keefe, a journalist for The New Yorker, shares little of the “tribal solidarity” with the old country that was ubiquitous in Boston, where he grew up.

“I never felt any particular interest in the conflict in Northern Ireland,” he writes. Whatever feeling he had about the Troubles — the vicious fighting between the mostly Protestant loyalists who wanted to remain British subjects and the mostly Catholic republicans who didn’t — amounted to a “detached concern.”

It’s a somewhat startling admission, coming toward the end of this resolutely humane book, but an outsider’s perspective is what gives “Say Nothing” its exacting and terrifying lucidity. The title comes from a poem by Seamus Heaney that describes “The famous / Northern reticence, the tight gag of place / And times.” Keefe’s book is as much about this “penumbra of silence” as it is about lives lost and blood shed.

The book begins with a longstanding mystery: Who abducted Jean McConville, and why? McConville was a mother of 10, born to Protestants and married to a Catholic, so overwhelmed by the daunting task of caring for her brood after her husband died that she seemed to have no time for anything else, much less sectarian intrigue. Yet in December 1972, at the end of the bloodiest year of the Troubles, a group of masked men and women barged into her Belfast home, dragging the 38-year-old widow away as her frightened children looked on.