-By Frank Lovece

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Starting strongly before petering out, and taking dead-end detours that feel like padding, this New York City documentary about a still-resonant, 1970s series of horrifying crimes against children washes over you with a heartbreaking setup, and then becomes a low-budget "60 Minutes"—a length that might have been preferable to avoid diluting its impact.

Filmmakers Joshua Zeman and Barbara Brancaccio were raised in Staten Island, the most New Jersey-like of the five boroughs, in the shadow of the Willowbrook State School. That infamously callous, overcrowded institution for mentally retarded children, which Senator Robert Kennedy in 1965 rightly labeled a "snake pit," was the subject of a local WABC-TV documentary in 1972: the Peabody Award-winning "Willowbrook: The Last Disgrace," by the young, up-and-coming reporter Geraldo Rivera. The power of that eye-searing documentary, a couple of minutes of which are excerpted here, contrasts with the heartfelt and stolid but ultimately underwhelming Cropsey, which aggregates well enough but adds nothing to the public record.

That record involves a spate of child kidnappings and killings in the 1970s, both before and after the slow closing of Willowbrook, which took from the 1975 signing of a consent decree that ended a class-action lawsuit, to 1987, when the last child was transferred from the hulking hellhole, itself abandoned and left to rot. During that time, well over a score of children vanished—only the remains of one were ever recovered. That was Jennifer Schweiger, a 12-year-old with Down syndrome, who was found in a shallow grave by a community volunteer, one of some 5,000, who'd chanced over a soft clay spot on the Willowbrook grounds where police had already been. Convicted was Andre Rand, an unstable former Willowbrook employee—described here as having been a physical therapist, which can't possibly be right; an assistant or a P.T. orderly maybe—who was living in makeshift hovels and campsites there.

The documentary questions whether Rand—who was convicted then of kidnapping and in 2004 convicted in the 1981 case of another missing mentally challenged girl, Holly Ann Hughes—was indeed guilty, or whether he was a convenient boogeyman targeted by community outrage. That would seem less a question had the documentary detailed a larger context that goes unmentioned except in parsimonious passing, namely Rand's record of doing this sort of thing: He'd served 16 months for a sexual-abuse charge after attempting to rape a Bronx nine-year-old in 1969, and was accused a raping a young woman and a 15-year-old girl, neither of whom pressed charges, in 1979. And while we do hear a young man's chilling story of a near-kidnapping in 1983, when Rand enticed 11 Staten Island children into his van and drove them to Elizabeth, NJ, and Newark Airport before returning them, we don't hear that he then spent 10 months in jail for unlawful imprisonment.

These details would have better served the story than do detours into Staten Island Satanism and an inexplicable nighttime trip to the abandoned Willowbrook. The documentary's title itself derives from a strained metaphor, with the filmmakers painting the tragic events as the embodiment of the Cropsey folk legend—that of the ax-wielding or perhaps hook-handed child-killer said to roam the local woods from the Hudson River Valley to Staten Island. That itself seems a dubious hook, and while the huge-hearted filmmakers themselves surely didn't intend to, the metaphor unwittingly makes light of the tragedies. Rand was Rand, not an urban legend, and the victims knew they were being taken by flesh-and-blood, not a phantom.