The execution : a Jeremy Fisk novel / Dick Wolf.

Enhanced descriptions from Syndetics:

NYPD Detective Jeremy Fisk--introduced in Law & Order creator Dick Wolf's New York Times bestselling debut The Intercept--must stop an assassin in the pay of a shadowy cartel in The Execution, a tense thriller that superbly blends suspense, politics, intrigue and high-flying action in the tradition of Vince Flynn, David Baldacci, and Robert Crais.

Ten days after the Mexican presidential election, twenty-three bodies are discovered beheaded on the United States border, each marked with a carving of a Hummingbird. Detective Cecilia Garza of the Mexican intelligence agency recognizes it: it is the signature of an assassin called Chuparosa. Garza has been pursuing the killer for years, yet knows little about him, except that he's heading to New York--with the rest of the world.

It's United Nations Week in Manhattan and Jeremy Fisk can't let grief over a devastating loss keep him from safeguarding his city. Complicating matters is the startling news of a mass murder in nearby Rockaway--and the arrival of a disturbingly beautiful and assertive Mexican cop.

To have a chance at finding Chuparosa, these uneasy allies must learn to work together and fast. As they soon discover, there's more to this threat than meets the eye--and that justice is not always blind.