Invisible Threat. The Aaron Holt Series, #5

Years ago, a classified black ops program called ASH ran quietly in the shadows of Eastern Europe, experimenting on human subjects, extracting memories, and pushing the limits of what was thought permissible. Then it was shut down. It was buried and forgotten. And then it wasn't. One by one, people connected to the project are turning up dead. Accidents, overdoses, and murders with no suspects. Over a dozen names were on the list, and now...
only a handful remain. Aaron Holt is one of them. Now Holt is racing across Europe, trying to reach the survivors before someone else does. Someone has a personal score to settle. Someone who's working through the list, with no intention of stopping until every last name on it is gone.