WASHINGTON — The Supreme Court, in a first-of-its-kind ruling, concluded unanimously today that murder trial spectators were free to wear buttons with a picture of the victim in front of the California jury that convicted the defendant.

The justices ruled in favor of California prosecutors who said the buttons were a harmless expression of grief by family members at the trial of Mathew Musladin.

It was the first time the justices ruled on whether the conduct of courtroom spectators deprived a defendant of a fair trial, which in this instance resulted in a conviction that was reversed by an appeals court.

The case Carey v. Musladin, 05-785, had First Amendment overtones but was not decided on First Amendment grounds.

Musladin had successfully challenged his conviction in the 1994 shooting death of Tom Studer, the fiancé of Musladin’s estranged wife, in San Jose, Calif. Musladin is serving a sentence of 32 years to life in prison.

Justice Clarence Thomas, writing for five of his colleagues, said the buttons “did not deny [Musladin] his right to a fair trial,” although he said the question remains open whether spectators’ conduct could be prejudicial. The other three justices also concurred with the Court’s judgment, but did not join Thomas’ opinion.

While the case was not decided on free-speech grounds, Justices David Souter and John Paul Stevens did invoke the First Amendment in their concurrences.

Souter wrote: “[A]n interest in protected expression on the part of the spectators wearing mourners’ buttons has been raised, but not given focus or careful attention in this or any other case that has come to our notice. Although I do not find such a First Amendment interest intuitively strong here, in the absence of developed argument it would be preferable not to decide whether protection of speech could require acceptance of some risk raised by spectators’ buttons.”

In his concurrence, Stevens said his reasons for joining the Court’s opinion were “essentially the same as those expressed by Justice Souter, with one caveat.”

“In my opinion,” he said, “there is no merit whatsoever to the suggestion that the First Amendment may provide some measure of protection to spectators in a courtroom who engage in actual or symbolic speech to express any point of view about an ongoing proceeding.”

Three members of Studer’s family wore a button with a photo of the victim throughout the trial over the objections of Musladin’s lawyer. The request prior to trial to preclude the family members from wearing the buttons was rejected by the judge, who said he saw “no possible prejudice to the defendant.”

But the 9th U.S. Circuit Court of Appeals ordered a new trial. The appeals panel said the buttons “essentially argue that Studer was the innocent party and that the defendant was necessarily guilty.” The dissenting judge in the 2-1 decision said the buttons were a symbol of a family’s grief.

The case turned on a 1996 law, the Antiterrorism and Effective Death Penalty Act, which limited the circumstances under which federal judges may grant habeas corpus petitions in state court cases.

In order for a federal judge to step in, says the law, the state court decision must be “contrary to” or an “unreasonable application” of clearly established federal law as determined by the Supreme Court.

The appeals court said the state court decision was contrary to Supreme Court holdings. But Thomas said, “We hold that it was not.”

Justice David Souter, who wrote a concurring opinion, said there can be little doubt that allowing the buttons at a murder trial “can raise a risk of improper consideration.” But he also said he could not conclude that the state court acted unreasonably.

The closest case to Musladin’s came six years before the death penalty act, when the San Francisco-based appeals court tossed out a rape conviction on grounds that jurors might have been prejudiced because some trial observers wore pins that read “Women Against Rape.”

The justices have ruled on prejudicial impact involving defendants, but never about spectators wearing buttons.

The Court has forbidden the use of visible shackles for defendants in front of juries unless the use is justified by an essential state interest such as courtroom security. The Court also has forbidden having defendants wear prison garb in front of juries in most instances.

The state of California argued that Musladin’s conviction did not violate clearly established law and was not an unreasonable application of the law.