A claim that implicates two men in a 1975 murder that sent Kennedy cousin Michael Skakel to prison is "pretty compelling," a Connecticut Supreme Court justice said Thursday, but justices also expressed skepticism.

Justice Richard N. Palmer pressed prosecutors about why they didn't grant Gitano "Tony" Bryant immunity when he implicated two men in Martha Moxley's death a year after Skakel was convicted. Bryant's videotaped account is key to Skakel's request to overturn his 2002 murder conviction.

"It's pretty compelling, I think," Palmer said during a state Supreme Court hearing Thursday.

Prosecutor Susann Gill said Bryant was not a credible witness. None of the 15 people he claimed to have seen the night of the murder — including Skakel's siblings — corroborated his presence, she said.

"The state is not in the habit of granting immunity to people whose credibility we think is worthless," Gill said.

Skakel, a nephew of Ethel Kennedy, was sentenced to 20 years to life in prison for fatally beating Moxley with a golf club when they were 15.

Thursday's hearing was an appeal of a 2007 decision by Stamford Superior Court Judge Edward R. Karazin Jr. that denied Skakel a new trial.

Bryant and Skakel attended the same private school as children. In his videotaped statement to an investigator hired by Skakel, Bryant said his two friends were in Greenwich the night Moxley was killed. He said they told him they got Moxley "caveman style."

Bryant has since invoked his Fifth Amendment right against self-incrimination. The two men he implicated have done the same.

Prosecutors have said Bryant's claim was fabricated and that nobody saw him and his friends in the predominantly white, gated neighborhood the night of the murder. Bryant and one of the men he implicated are black; the other has been described as mixed race.

Palmer and other justices on Thursday questioned why nobody saw Bryant's friends in Greenwich on the night of the murder. One justice said the men would have been hard to miss.

One of Skakel's attorneys, Hubert Santos, blamed the Greenwich police investigation.

But Palmer challenged that argument.

"It's hard for me to believe the Greenwich police could have been that slothful in ascertaining who was in the neighborhood at that time," Palmer said.

Palmer also pressed prosecutors on why Bryant would make up the story. Gill called it "I Love Lucy syndrome." Coined by Skakel's trial attorney, Michael Sherman, it refers to people who want to "get into the act" in the high profile case.

But Palmer said Bryant seemed to be a reluctant witness.

"That's somewhat counterintuitive to me that anybody would do this," Palmer said. "It's hard for me to fathom."

Skakel's attorneys also accused state investigator Frank Garr of bias because of a book they say he planned with a former Newsday reporter. Garr said he did not get involved in the book until after the verdict.

Santos said Garr was paid $7,000 for his role in the book. He said the book deal would have opened a whole new line of defense: that Garr's interaction with witnesses made their testimony unreliable.

The court did not indicate when it would make a decision.

Skakel lost an appeal before the state Supreme Court in which he argued, among other things, that the statute of limitations had expired when he was charged in 2000. He has an appeal pending in federal court.

Dorthy Moxley, the victim's mother, attended the hearing and said she remains convinced of Skakel's guilt.

"You do get a little anxious at the thought maybe you'll have to go through another trial," Moxley said. "I know he will get out some day, but I'd like it to be later rather than now."

Skakel's brother Stephen said he remains confident his brother will be vindicated. He said he considers Bryant's claim credible.

"Why would any one in their right mind put themselves in that situation?" Skakel said.