They were U.S. Arsenals. The U.S. government could issue arms and equipment to the states, but they were federal facilities. The state may have had its own arsenal, but these were U.S. property. Part of the old St. Louis Arsenal was deeded to the City of St. Louis in the 1870's and made into a park (Lyon Park actually), but the rest of the property is a U.S. government installation to this very day. As an aside, although it isn't open to the public, you can still see some of the antebellum wall and buildings from the park.