The case had raised alarms among civil libertarians, who said it threatened the rights to free speech and protest.

In an unsigned opinion, the justices agreed the ruling threatened the 1st Amendment, and they told the Louisiana Supreme Court to consider whether such a “novel” claim might proceed under state law. A lawyer for the BLM leader said he was gratified the appeals court ruling had been wiped away.

The court also revived the lawsuit of a Texas prisoner who was held in a pair of “shockingly unsanitary” cells. A second panel of conservative 5th Circuit judges had granted the prison guards’ claims of “qualified immunity,” which prevented them from being sued.

“It gives me great pleasure on behalf of myself and my colleagues to welcome Justice Barrett to the court,” said Chief Justice John G. Roberts Jr. At another time, and presumably when the pandemic has waned, the high court will hold an invocation ceremony to formally welcome her, he said.

The justices spent the next hour struggling over the Freedom of Information Act, which requires federal agencies to reveal records of their decisions but shields draft memos that are considered part of their internal deliberations.

Barrett spoke last but agreed with a Justice Department lawyer who was battling the Sierra Club over whether the U.S. Fish and Wildlife Service must reveal memos questioning an Obama-era regulation that would have allowed water-cooling systems that kill fish.

She said the court could adopt “some sort of bright line” that shields all the internal documents that come before a final decision. “Yes, your honor, I think that captures the way that we think the case ought to be resolved,” the government lawyer said.

Barrett took no part in the decision and orders issued Monday.

In the lawsuit by the injured Baton Rouge, La., police officer, identified only as John Doe, he did not allege that civil rights activist DeRay Mckesson led, directed, encouraged or participated in the violence or the assault on the officer. Rather it said only that he was “in charge of the protests” and “did nothing to calm the crowd.”

Mckesson led a rally in July 2016 several days after Alton Sterling, a 37-year-old Black man, was shot and killed by two police officers who tackled him outside a convenience store.

The Black Lives Matter rally blocked a highway in front of the police station, and officers in riot gear stood outside to confront the crowd. Some protesters began hurling water bottles, and an identified person threw a piece of concrete or a rock that struck an officer in the face. He suffered a brain injury, a loss of teeth and a broken jaw. More than 100 people, including Mckesson, were arrested.

The officer filed a lawsuit seeking to hold Mckesson liable for his injuries. A federal judge initially dismissed the suit, but last year, to the surprise of civil libertarians, a three-judge panel of the 5th Circuit Court revived the suit.

These judges noted the protesters were illegally blocking a highway. “We ultimately find that Mckesson’s conduct at this pleading stage was not necessarily protected by the 1st Amendment,” wrote Senior Judge E. Grady Jolly, a Reagan appointee. “We perceive no constitutional issue with Mckesson being held liable for injuries caused by a combination of his own negligent conduct and the violent actions of another that were foreseeable as a result of that negligent conduct.”

The American Civil Liberties Union called the ruling a threat to protests nationwide and urged the justices to intervene to block the suit from going forward in Louisiana courts.

“If the law had allowed anyone to sue leaders of social justice movements over the violent actions of others, there would have been no civil rights movement,” said David Cole, the ACLU’s legal director.

The justices considered the appeal for several weeks and issued an unsigned ruling on Monday that “vacated” the appeals court ruling and asked the Louisiana courts to consider whether the police officer’s suit could proceed under state law.

“Under the unusual circumstances we confront here, we conclude that the 5th Circuit should not have ventured into so uncertain an area of tort law — one laden with value judgments and fraught with implications for 1st Amendment rights — without first seeking guidance on potentially controlling Louisiana law from the Louisiana Supreme Court,” the court said in Mckesson vs. Doe.

The justices noted that it was not clear under state law that Mckesson had violated any “duty” that would subject him to a lawsuit. It was also uncertain whether a police officer may sue for injuries that occurred when he was acting in the line of duty.

Justice Clarence Thomas dissented but did not say why.

Mckesson and his lawyers called the ruling an important win but not a final victory. “I’ve been fighting this case for four years. Today’s decision recognizes that holding me liable for organizing a protest because an unidentifiable person threw a rock raises 1st Amendment concerns. I’m gratified that the Supreme Court vacated the ruling below, but amazingly, the fight is not over.”

His lawyer David Goldberg said, “Today’s action by the Supreme Court represents an important step in vindicating the right to protest — the rights of all Americans to speak, assemble and petition their government. And it is a significant defeat for those who seek to use the threat of damages lawsuits to silence the voices of people of modest means who participate in our government by taking to the streets to have their voices heard.”

The case drew attention after an unusual switch in positions by Judge Don R. Willett, a Trump appointee who was listed as a potential Supreme Court nominee. He had at first joined the 3-0 decision appellate written by Jolly but changed his mind and issued a dissent a few months later.

“I have had a judicial change of heart. Further reflection has led me to see this case differently,” he wrote. “As Justice [Felix] Frankfurter elegantly put it 70 years ago, ‘Wisdom too often never comes, and so one ought not to reject it merely because it comes late.’”

He said, “Officer Doe risked his life to keep his community safe that day — same as every other day he put on the uniform. He deserves justice. Unquestionably, Officer Doe can sue the rock thrower. But I am unconvinced he can sue the protest leader.”

The dissent prompted a move to have the panel’s decision reconsidered by the full 5th Circuit Court, but it failed on a tie vote of 8-8.

The Texas prisoner’s case was Taylor vs. Riojas. As described by the court, Trent Taylor was first held in a cell that was “covered, nearly floor to ceiling, in massive amounts of feces,” including on the window, walls and even packed inside the water faucet. Fearing his food and water would be contaminated, he did not eat or drink for nearly four days, court documents say. The guards then moved him to a second, “frigidly cold cell, which was equipped with only a clogged drain in the floor to dispose of bodily wastes.”

The 5th Circuit in an opinion by Judge Jerry Smith said the prison guards were immune from being sued because they did not have “fair warning” these conditions were inhumane and unconstitutional.

The high court disagreed. “No reasonable correctional officer could have concluded that, under the extreme circumstances of this case, it was constitutionally permissible to house Taylor in such deplorably unsanitary conditions for such an extended period of time,” the justices said.

Thomas dissented alone without comment.

“Today’s ruling is a big victory — most importantly for Trent Taylor, but also in the fight against qualified immunity,” said Brianne Gorod, chief counsel for the Constitutional Accountability Center. “The sad fact is that Taylor’s experience, while almost unspeakably horrible, is not unlike the experiences that many others have suffered and continue to suffer at the hands of law enforcement.”