The Rithian Terror

Republishing all of SFWA Grandmaster Damon Knight's books, in conjunction with the author's family - see all of Damon Knight's books from ReAnimus Press!A psychological thriller that follows an Earth security officer in the future who is racing against time to locate an alien spy. This is one of the first books to feature a surveillance drone.
In the year 2521, the xenophobic Earth is the dominant power in a Galactic empire, with a policy of ruthlessly conquering or undermining other alien races it encounters. The latest are the Rithians, and after some years of clandestine harassment by Earth, the Rithians have managed to place a group of spies disguised as humans on the Earth.
How do you find a space spy who substitutes perfectly for anyone in the universe?
The fate of the Earth Empire hangs in the balance-and Security Commissioner Spangler knows it's up to him to find the monster, the Rithian Terror. Seven Rithians had landed on earth. Six had been disposed of. One was loose. It could look like anyone. Can Spangler find the spy in time? Can the Empire survive? Should it?