The complaints may be public record, but they're probably not available without a written request, fee, etc.
I did find this within less than 2 minutes of google searching:
John Burris, an Oakland-based civil rights lawyer representing Grant’s family, says he believes Mehserle quit in order to avoid being interviewed by investigators in his department. He added that BART legally could have demanded that Mehserle and the other BART police officers at the scene give statements on the night of the incident. So far, they have not demanded that he give a statement.
Burris also said there had been at least two prior complaints about Mehserle before the shooting.
http://510report.org/2009/01/08/hundreds-protest-bart-shooting/