Baen, 1986 [1961,1962]
Synopsis: The U.S.S. Benjamin Franklin has returned from a three-year mission to the galactic core to find the Earth utterly lifeless, far beyond the current known abilities of humans. The obvious suspects are some species of intelligent extra-terrestrial, with whom the Earth has been in contact since the 1980s. There are a number of suspects in this SF mystery, from the imperialistic Kandemiri to some group of humans using extra-terrestrial technology.