In a last-ditch legal move, defense attorneys petitioned the high court earlier this month, alleging shoddy forensic testing and other errors may have wrongly sent Williams to San Quentin State Prison, where he is scheduled die by injection Dec. 13.
"It's a horrific crime, because of what he's been convicted of. We assume he's guilty but there are circumstances about this person that warrant mercy," Ellen Kreitzberg, head of Santa Clara University's Death Penalty College, told radio station KCBS-AM.
Lawyers for Williams, author of a series of anti-gang books for children, wanted to re-exam ballistics evidence that showed his shotgun was used to kill three people during a 1979 motel robbery in 1979.
The defense claimed the forensic evidence was "junk science," but prosecutors said that allegation was "based upon innuendo, supposition and the patent bias of (Williams') purported expert."
"The extraordinary relief Williams sought is reserved for those cases which have legal merit," said Nathan Barankin, spokesman for Attorney General Bill Lockyer.
The high court voted 4-2 without comment to deny the inmate's petition, with Chief Justice Ronald George voting to reopen the case.
"We think the chief justice's dissent highlights the seriousness of the issues raised," defense attorney Jonathan Harris said. He was unsure whether he would ask the federal courts to intervene again.
Gov. Arnold Schwarzenegger could also still intervene. He has agreed to hear Williams' clemency petition, and if clemency is granted it would commute the inmate's sentence to life without parole.
"What I want to do is make sure we make the right decisions, because we're dealing here with a person's life," Schwarzenegger said Wednesday.
The high court's ruling came as death penalty opponents rallied around the state urging the governor to spare Williams' life because of his apparent turnabout on death row.
"We're all remaining optimistic. We're all remaining prayerful," Bonnie Williams-Taylor, Williams' ex-wife and the mother of one of his sons, said at a rally in Los Angeles.
Williams, condemned in 1981, has maintained his innocence. Among his claims is that fabricated testimony sent him to death row. He also says prosecutors violated his rights when they dismissed all potential black jurors from his case.
The California Supreme Court, federal trial and appeals courts, and the U.S. Supreme Court have already ruled against him in earlier appeals.
Williams was condemned for killing Yen-I Yang, Tsai-Shai Chen Yang and Yu-Chin Yang Lin in the motel robbery, and for gunning down Albert Owens, a 7-Eleven clerk, in a separate crime.
While in prison, Williams has campaigned for an end to youth gang violence while co-authoring anti-gang books for youngsters. Supporters have nominated him several times for the Nobel Peace Prize.
Williams and a high school friend started the Crips in Los Angeles in 1971 and it grew into one of the nation's most notorious street gangs.