U.S. authorities try to solve the puzzle that a vengeance-seeking terrorist leaves behind in Washington, D.C., in this thriller.

FBI Special Agent and physician Kate Morgan is riding a commuter train to a congressional hearing. But she’s lucky to be alive after an explosive obliterates the tracks and derails the train. Readers know immediately that a man named Phillip Barnes is responsible for the attack, but his motives aren’t entirely clear. He definitely has a vendetta against Kate, whom he blames for killing his family. Inside a briefcase marked with Kate’s name, Barnes leaves a partial crossword puzzle and corresponding clues. Feds hope that deciphering the puzzle will lead them to the site of the next attack before it happens. Meanwhile, the Pentagon sends U.S. Army Capt. Rachel Pratt to assist the FBI’s investigation, though her secret directives include locating and eliminating the threat. But once she identifies the culprit as Barnes, with whom she has a personal history, Pratt ignores orders to return to her post and continues hunting the terrorist. As more pieces of the crossword arrive, feds bring in puzzle-solving guru (and civilian) Will Shortz for assistance. Kate begins to suspect that the strike in Washington relates to a covert program involving experiments on American soldiers. She’s likewise certain that Pratt, though helpful, is withholding information from her. Nevertheless, neither woman is safe, as Barnes’ plan, after he toys with the unwitting participants in his game, is sure to have a lethal ending. Grant (The Singularity Witness, 2018) fills this narrative with superlative female characters. Kate, for one, who appeared in the author’s preceding novel, is as smart as she is capable. She decrypts many of Barnes’ clues on her own and survives more than one explosion through sheer determination. There’s also equally intelligent Strategic Information & Operations Center unit chief Alice Watson, and resourceful, enigmatic Pratt. A surprising standout character among the men is Shortz, a fictionalized version of the real-life puzzle master. Though he’s unaccustomed to someone pointing a gun at him, he keeps his cool when it inevitably occurs. Grant retains a steady pace and suspense as even the reverberations of bombings elicit lasting images: “A heavy sky had turned orange, almost bloodlike in spots. Fire licked at building openings, places where doors and windows had been.” What exactly is unfolding, especially specifics on the revenge Barnes seeks, is a mystery for much of the story. But it’s the characters that truly generate the narrative tension. For example, Kate is understandably wary of Pratt, who’s sometimes deceitful and typically evasive. This ultimately leads to a gleefully convoluted face-off with Kate and several other characters that sizzles with revelations, double crossings, and seemingly shifting alliances. Some things are left unresolved by the end, like a mystery possibly involving neuroscientist Thomas Parker, who despite starring alongside Kate in the earlier book doesn’t show up here. But Grant thoroughly and convincingly wrap ups this story’s main plot.

A taut, exhilarating mystery with plenty of material for a sequel.