How have the courts dealt with allegations that the police knew that a witness could testify to the defense?

The petition and supporting affidavits charged that the police knew that a certain witness could testify to facts material to the defense and that the evidence had been suppressed. The return and counteraffidavits establish to our satisfaction that neither the police nor the district attorney knew of the existence of the witness or the evidence, whereas defendant's attorney had sufficient information to put him on inquiry. In addition, the evidence which was assertedly suppressed appears to be consistent with the theory of the prosecution and was not such as would have prevented entry of the judgment. (Cf. People v. Tuthill, 32 Cal.2d 819, 821-822, 827, 198 P.2d 505.)