How have courts excluded evidence of third party culpability in a murder trial?

Instructive is People v. Sandoval (1992) 4 Cal.4th 155, where the court held evidence of third party culpability was properly excluded. The defense had sought to prove the murder victim was the center of a violent criminal operation involving drugs, stolen cars, and guns, and that any number of accomplices or rivals could have killed him. In support of this theory, the defense sought to cross-examine a police officer about the names found in the victim's appointment book and weekly planner and to introduce that document into evidence. The court held this evidence only raised the possibility that others had a motive to kill the two victims. Although the defense had identified two people who had a plausible motive, there was no direct or circumstantial evidence linking them to the actual perpetration of the crime. (Id. at pp. 176.)