Not aliens. We are told that they are made by magnetars, a type of neutron star:
We didn’t even know that FRBs existed until 2007 but have since cataloged hundreds of them; some come from sources that repeatedly emit them, while others seem to burst once and go silent …
Those suspicions have now been borne out, as astronomers have watched a magnetar in our own galaxy sending out an FRB at the same time it emitted pulses of high-energy gamma rays. This doesn’t answer all our questions, as we’re still not sure how the FRBs are produced or why only some of the gamma-ray outbursts from this magnetar are associated with FRBs. But the confirmation will give us a chance to look more carefully at the extreme physics of magnetars as we try to understand what’s going on.
John Timmer, “We finally know what has been making fast radio bursts” at Ars Technica
See also: Harvard Astronomer Is Back: Radio Bursts Could Be Aliens