Is there a real agency like NCIS?

Yes, the Naval Criminal Investigative Service (NCIS) is a real federal law enforcement agency in the United States. NCIS is a civilian law enforcement agency within the Department of the Navy, responsible for investigating criminal activities involving the U.S. Navy and Marine Corps. Its primary mission is to investigate major crimes, such as murder, terrorism, espionage, sabotage, fraud, and drug trafficking, that occur within the Navy and Marine Corps, as well as provide protective services to Navy and Marine Corps personnel. NCIS has jurisdiction over all Navy and Marine Corps installations worldwide, as well as over U.S. Navy and Marine Corps personnel assigned to duty at other locations around the world. NCIS agents are sworn federal law enforcement officers with the authority to carry firearms and make arrests.