WASHINGTON — John Fry was convicted of shooting two people to death alongside a Northern California highway even though a witness overheard another man confess to the crimes.

On Tuesday, the Supreme Court debated whether a trial judge’s decision to exclude the witness’ testimony warrants overturning Fry’s conviction, which resulted in a life term in prison.

Fry’s lawyer argued that the state trial court had significantly violated his client’s constitutional rights by barring Pamela Maples’ testimony against her cousin, Anthony Hurtz.

Maples was not located until Fry’s trial was already under way.

Ross Moody, a deputy attorney general from California, said that although Maples was perhaps more credible than the witnesses who actually testified against Hurtz, she wasn’t even sure what murders she heard him talking about or if it was a serious discussion. Other witnesses against Hurtz were mostly his ex-girlfriends and their relatives.

Keeping out Maples’ testimony was “the type of error that can result in the conviction of an innocent man,” said Fry’s court-appointed attorney, Victor Haltom.

Two justices suggested that Haltom might be right.

“I cannot accept the conclusion that there was no possible prejudice,” said Justice David Souter. “I just don’t find that a reasonable conclusion.” Justice Stephen Breyer also questioned the conclusion that the error was harmless.

State courts said the judge’s decision was proper. A divided three-judge panel of the 9th U.S. Circuit Court of Appeals ruled that the judge was wrong to bar “reliable” evidence. However, the appeals court said the error was harmless because it would not have substantially affected jury deliberations.

Fry was convicted on his third trial, after two earlier juries deadlocked. The Solano County Superior Court panel that convicted him deliberated for five weeks, at one point declaring itself deadlocked.

That was enough to suggest to several justices that Maples’ testimony could have made a difference.

But Patricia Millett, a Bush administration lawyer, said five courts previously reviewed the case and none determined that Fray was denied a fair trial. “Close calls go to the state and are not overturned by the sixth court on review,” Millett told the justices.

Moody said Maples’ testimony would not have added much to the deliberations

“This witness may have been Mr. Hurtz’s cousin and not his ex-girlfriend or his ex-girlfriend’s mother, but she did not have very much to say about this,” said Moody.

Chief Justice John Roberts was sympathetic to that.

“How strong is this witness who didn’t even know if it was a serious conversation?” he asked.

At issue in the case is what standard of review should apply to determine if a defendant’s constitutional rights were violated, and whether state and federal courts should adhere to the same standard.

According to trial testimony Fry sold methamphetamine to one of his victims, Cynthia Bell, and threatened to kill her over a drug debt shortly before she and James Bell turned up dead. Hurtz also was a drug dealer.

The case is Fry v. Pliler, 06-5247.