The incident raises questions about the effectiveness of Russia's air defense system. The S-400 system is capable of intercepting missiles up to 16 kilometers away.
It is designed to protect Moscow's decision-making center from air and sea attacks. Russia has banned the use of drones in its airspace since the start of the year. The U.S. government says it is working with Russia on a new system to protect the U.N. secretary-general's office from drone attacks in the future.