For a generation of kids on Staten Island, "Cropsey" was the go-to bogeyman, identified in campfire tales as a former resident of the island's abandoned mental hospital who now haunted the woods, waiting to snatch and kill children who dared to disobey their parents. The Cropsey legend took a horrifying twist in the late '80s, when a 13-year-old girl with Down syndrome went missing and turned up in a shallow grave, with all signs pointing to a real-life Cropsey: a former custodian at the mental institution who sporadically lived in the woods and was eventually convicted of the crime.

In their documentary Cropsey, filmmakers and Staten Island natives Joshua Zeman and Barbara Brancaccio dive into the Cropsey saga in classic This American Life style, blending personal reminiscences with interviews of police, victims' families, and fellow Cropsey obsessives. Zeman and Brancaccio make appealing guides, and for the first hour or so, Cropsey delivers on its brainy, creepy promise, thanks primarily to the powerful facts: Due to a career-making exposé by a young Geraldo Rivera (portions of which are featured here), the Staten Island mental institution was revealed to be a truly hellish operation, a dumping ground for the severely developmentally disabled of all ages, who were stripped naked, chained to walls, and left to scream in their own filth. The idea that such a place might pollute the soil with evil for decades to come is presented with appealing clarity.

But things get fuzzy as Zeman and Brancaccio overplay the "alternate theories" of the alleged Cropsey killings. Determined to send viewers out of the cinema pondering the Mystery of Cropsey, the filmmakers devote significant screen time to negligible gossip, which does an okay job of capturing the spooky moods of Staten Islanders but does little to stoke the mystery. Despite these stumbles, Cropsey earns its screen time as a portrait of a borough obsessed with evil.