  As a major approach to looking for life beyond the Earth, the search for
extraterrestrial intelligence (SETI) is committed to detecting technosignatures
such as engineered radio signals that are indicative of technologically capable
life. In this paper, we report a targeted SETI campaign employing an
observation strategy named multi-beam coincidence matching (MBCM) at the
Five-hundred-meter Aperture Spherical radio Telescope (FAST) towards 33 known
exoplanet systems, searching for ETI narrow-band drifting signals across
1.05-1.45 GHz in two orthogonal linear polarization directions separately. A
signal at 1140.604 MHz detected from the observation towards Kepler-438
originally peaked our interest because its features are roughly consistent with
assumed ETI technosignatures. However, evidences such as its polarization
characteristics are almost able to eliminate the possibility of an
extraterrestrial origin. Our observations achieve an unprecedented sensitivity
since the minimum equivalent isotropic radiated power (EIRP) we are able to
detect reaches 1.48 x10^9 W.
