True-crime documentaries tend to focus on forensics, ballistics, transcripts and other evidence to lead viewers to conclusions, but “Strong Island,” by Central Islip native Yance Ford, hinges on something else: a feeling.

That’s exactly what the modern legal system was designed to weed out, of course, but it’s that very push-pull between empirical facts and a nagging knot in the gut that makes this first-person documentary so compelling. Directed and narrated by Ford, it’s a re-examination of the case of his oldest sibling, William, a 24-year-old black schoolteacher who was fatally shot by a white car mechanic, Mark Reilly, 19, during an argument in April 1992. Though William was unarmed, Reilly claimed self-defense, and an all-white grand jury declined to send his case to trial. The case files remain sealed, but it’s clear to Ford and his family that racial bias robbed them of justice 25 years ago.

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That isn’t decisively provable and William’s story is not a tidy one. Ford uses interviews with his mother, Barbara; two of William’s friends; and himself to show that his brother was not the rage-fueled monster described by Reilly. Indeed, William aspired to be a policeman, and we learn that he once heroically chased down a mugger who shot a man at an ATM. We also learn, however, that William had previously visited the auto-body shop where Reilly worked and — in a dispute stemming from a car repair — became belligerent, allegedly throwing a vacuum cleaner. Does that justify his shooting? A grand jury apparently thought so. Describing that trial, Barbara — a plain-spoken, forceful and highly convincing figure — says the white jurors read magazines and finished crosswords during her testimony.

“Strong Island” is a highly personal and even poetic film, which is both its strength and its weakness. Ford’s speeches to the camera — in unsparing, extreme close-ups — are sometimes rivetingly emotional, sometimes a tad too eloquent. The shots of Long Island streets, shown upside-down from a moving car, are visually arresting but perhaps over-symbolic: a world upended. William, the film’s central figure, can get a little overshadowed by the filmmaker’s artistic choices.

“Strong Island” walks right up to us and asks a hard question: If William and his killer traded skin color, how would this story end? We may not have concrete proof, but we’re pretty sure we know the answer.