I Let You Go by Clare Mackintosh
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Officially, Thomas Laker is an employee of the NSA. His real employer, known as the Gray Outfit, is not listed--anywhere. When a Washington, D.C., insider is killed in a bomb explosion, Laker teams up with cryptographer Ava North on a desperate search for clues. The only thing certain is that another act of terrorism is imminent. Delving into the dangerous past when America was drawn into global conflict, they discover one of history's greatest--and deadliest--secrets. In the wrong hands it can unleash unimaginable destruction. Now, to keep his homeland from plunging into its darkest hour, Laker will have to defend everything he believes in.