We study a sample of gamma-ray bursts detected by the Swift satellite with known redshift which show a precursor in the Swift BAT light curve. We analyze the spectra of the precursors and compare them with the time-integrated spectra of the prompt emission. We find neither a correlation between the two slopes nor a tendency for the precursors spectra to be systematically harder or softer than the prompt ones. The energetics of the precursors are large: on average, they are just a factor of a few less energetic (in the source rest-frame energy range 15-150 keV) than the entire bursts. These properties do not depend on the quiescent time between the end of the precursor and the start of the main event. These results suggest that what has been called a "precursor" is not a phenomenon distinct from the main event, but is tightly connected with it, even if, in some case, the quiescent time intervals can be longer than 100 s.
