As the officers tacked up posters and handed out fliers last July, a van with loudspeakers inched through Washington Heights, announcing yet another attempt by the police to dredge up anything resembling a clue in an emotionally wrenching case from 1991: the little girl whose emaciated and bound body had been left in a cooler by a highway in Upper Manhattan.

There was no name. No known family. No suspects.

After 22 years, it had become one of those cases that seem destined to go unsolved, no matter how detectives tried to jog people’s memories or find something that had eluded them the last time.

But that on-the-ground effort in July produced a tip. A woman recalled a conversation, years ago, in which another woman spoke of a younger sister murdered. She did not know if the dead girl was the one the police called Baby Hope, but the similarities were apparent.

The lead was pursued, and the older sister was found. Investigators then tracked down the woman’s mother, surreptitiously taking a sample of her DNA.