Sci-fi Radio (ep18) Impostor by Philip K. Dick

Impostor by Philip K. Dick
Spence Olham, a member of a team designing an offensive weapon to destroy invading aliens known as the Outspacers, is confronted by a colleague and accused by security officer Major Peters of being an android impostor designed to sabotage Earth's defenses. The impostor's ship was damaged and has crashed just outside the city. The android is supposed to detonate a planet-destroying bomb on the utterance of a deadly code phrase.
Olham, in an attempt to clear his name and prove his humanity, manages to escape his captors and return to Earth after they fail to kill him on the Moon. Upon reaching Earth, Olham contacts his wife, Mary, but is soon ambushed by security officers waiting for him by his house. Out of options and with Major Peters' forces closing in, Olham decides to prove he is a human by finding the crashed Outspacer spaceship and recovering the android's body from the wreckage.
The discovery of a bloody knife by the wreckage indicates to Olham that Peters was correct and that the real Spence Olham had already been killed. The android, now aware of the truth of its existence, proclaims "If that's Olham, then I must be..." causing the bomb to detonate, the explosion visible even to the Outspacers of Alpha Centauri.