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The robber fled through a backyard in Queens, dropping a jacket and gloves and with them, genetic material.

Detectives failed to find a suspect in tests comparing the material against a DNA database of convicted criminals. But the search yielded a near match to a convict with an unusually similar DNA profile. The convict may not have been the robber, but perhaps one of his relatives was.

The process of turning crime-scene DNA into a family tree of possible leads has been quietly undertaken in more than two dozen cases in New York City since 2009, when a divided state committee voted to allow the release of so-called partial match DNA data to investigators.

Three years in, the street-level experience in New York has been decidedly mixed: Law enforcement officials said they knew of no cases solved in New York City because of a lead generated by a partial match.

The reason, investigators suggest, may be because New York — unlike states like California, Colorado and Virginia — has not allowed authorities to use existing software that explicitly searches state...