Books in the Heroes of the Empire series

Earthlings have colonised much of its arm of the Milky Way galaxy and has had a long era of peace. Suddenly, a small ship of explorers was attacked near the rim of Earth's empire and everyone was killed without damaging the ship. A patrol craft which was sent to investigate was similarly attacked. The attacker was invisible to any tracking device and had drained the life of all living organisms in the ships. When attacks began on populated planets, military action was ruled out to prevent the situation from getting worse. Instead, an experienced special agent on the other side of the empire was recalled from his vacation and sent to resolve the issue. This begins the story of this agent's gruelling mission to determine what the attacker was and stop it from killing billions of lives.