The disparate victims are spaced across America with scarcely enough to connect them to each other. An estate agent and her punctilious client; an elderly wheelchair-bound man and his daughter; the man painting their apartment; a dysfunctional family, including baby and visiting friend. The modus operandi speaks of a linkage - all handcuffed behind their backs, mouths taped shut, a single bullet hole through the back of the head. But investigating agencies are too well spread, too overwhelmed. No robbery, no sexual element. The single real clue is the Nike trainer seen by a single young survivor.

Interspersed with the brutality, author Dibdin weaves his second strand. Going to pick his son David up from a children's party, teacher Phil finds the place in chaos, the kids terrified. The host is gone, so too his son. Later, blood-stained clothing is found in a forest. The boy's dejected mother cracks under the strain and kills herself. Alone, grieving for his dead family, Phil decides to take up an offer to visit from his old college buddy Sam, but soon comes to realise that the religious fervour Sam showed in school was as nothing to the fully-fledged zealotry he finds on an island near the Canadian border.

It's a real shame. Up to now the book has pulled in two directions at once and pulled it off with aplomb. It's a style not unlike that of the masterful Thomas Harris - sacrificing mystery for the careful tension of a double narrative. A why rather than a how.

Dibdin isn't pretending these stories are somehow unconnected and the rationale for what's occurring - the explanation of the title itself, the twisted logic of God's (Sam's) plan - is superb.

It's when we switch in the final third to full-blown Manson, Jim Jones, Solar Temple, Koresh - even Oklahoma City bomb - territory that Dark Spectre loses it. There is something inevitable about the way the narrative is developing by now, and we wait in vain for Dibdin to pull the whole back from the abyss. Suddenly clever is out the window and we are expected to settle for page on page of people running about, gunfire. The really good stuff in here - Sam's mind blown on a cocktail of psychedelic overdose and his own personal sanctified Vietnam; his grotesque, corkscrew spiritual take on the poetry of nutty old William Blake - is terrific. The confused action-movie resolution is not.

Dibdin's previous thrillers featuring Venetian police detective Aurelio Zen have elevated Dibdin-stock to near the pinnacle of a British form more usually dominated by the twin female peaks of James and Vine/Rendell. Crime thriller as genuine literature. Dark Spectre feels like a push towards America, to break himself there, and he can hardly be blamed for that. Unfortunately, if crime writing is about chaos and order (which it surely is) it is also a fact that the best of the form sides forever with the chaotic. Two thirds of this book is truly feverish, darkly violent, driven by purely insane rationale. The remainder is a script for a neatly ordered, safe, half-decent TV mini-series, and therein lies our disappointment.