What is the test for establishing that counsel failed to investigate and/or failed to call witnesses?

Where the contention is that counsel failed to investigate and/or failed to call witnesses, "[t]he petitioner must demonstrate that counsel knew or should have known that further investigation was necessary, and must establish the nature and relevance of the evidence that counsel failed to present or discover." (People v. Williams, supra, 44 Cal.3d at p. 937.) "[T]he petitioner must not only prove that counsel inexcusably failed to make particular investigations . . . , or failed to introduce particular items of evidence, but must also demonstrate that the omissions resulted in the denial of or inadequate presentation of a potentially meritorious defense." (Id. at p. 936.)