By Andrew Chung

(Reuters) -The U.S. Supreme Court on Thursday refused to clear a path for victims of attacks by militant organizations to hold social media companies liable under an anti-terrorism law for failing to stop the groups from using their platforms, handing a victory to Twitter Inc.

The court in a separate case involving Google LLC sidestepped a bid to weaken legal protections for internet firms.

The justices in a 9-0 decision reversed a lower court's ruling that had revived a lawsuit against Twitter by the American relatives of Nawras Alassaf, a Jordanian man killed in a 2017 attack during New Year's celebration in a Istanbul nightclub claimed by the Islamic State militant group.

The case was one of two that the Supreme Court weighed in its current term aimed at holding internet companies accountable for contentious content posted by users - an issue of growing concern for the public and U.S. lawmakers.

The justices on Thursday, in a similar case against Google LLC-owned YouTube, part of Alphabet Inc, sidestepped making a ruling on a bid to narrow a federal law protecting internet companies from lawsuits for content posted by their users - called Section 230 of the Communications Decency Act.

The justices, in a brief and unsigned ruling, returned to a lower court a lawsuit by the family of Nohemi Gonzalez, a 23-year-old college student from California who was fatally shot in an Islamic State attack in Paris in 2015. The lower court had thrown out the lawsuit.

The Istanbul massacre on Jan. 1, 2017, killed Alassaf and 38 others. His relatives accused Twitter of aiding and abetting the Islamic State, which claimed responsibility for the attack, by failing to police the platform for the group's accounts or posts in violation of a federal law called the Anti-Terrorism Act that enables Americans to recover damages related to "an act of international terrorism."

Twitter and its backers had said that allowing lawsuits like this would threaten internet companies with liability for providing widely available services to billions of users because some of them may be members of militant groups, even as the platforms regularly enforce policies against terrorism-related content.

The case hinged on whether the family's claims sufficiently alleged that the company knowingly provided "substantial assistance" to an "act of international terrorism" that would allow the relatives to maintain their suit and seek damages under the anti-terrorism law.

After a judge dismissed the lawsuit, the San Francisco-based 9th U.S. Circuit Court of Appeals in 2021 allowed it to proceed, concluding that Twitter had refused to take "meaningful steps" to prevent Islamic State's use of the platform.

Conservative Justice Clarence Thomas, who authored the ruling, said the allegations made by the plaintiffs were insufficient because they "point to no act of encouraging, soliciting or advising the commission" of the attack.

"Rather, they essentially portray defendants as bystanders, watching passively as ISIS carried out its nefarious schemes," Thomas added.

President Joe Biden's administration supported Twitter, saying the Anti-Terrorism Act imposes liability for assisting a terrorist act and not for "providing generalized aid to a foreign terrorist organization" with no causal link to the act at issue.

In the Twitter case, the 9th Circuit did not consider whether Section 230 barred the family's lawsuit. Google and Meta's Facebook, also defendants, did not formally join Twitter's appeal.

Islamic State called the Istanbul attack revenge for Turkish military involvement in Syria. The main suspect, Abdulkadir Masharipov, an Uzbek national, was later captured by police.

Twitter in court papers has said that it has terminated more than 1.7 million accounts for violating rules against "threatening or promoting terrorism."

The case against Google involved the scope of a 1996 U.S. law called Section 230 of the Communications Decency Act, which provides safeguards for "interactive computer services" by ensuring they cannot be treated for legal purposes as the "publisher or speaker" of information provided by users.

The family had argued that YouTube provided unlawful assistance to the Islamic State, which claimed responsibility for the attack, by recommending the militant group's content to users.

In their brief ruling on Thursday, the justices wrote that they "decline to address the application of (Section 230) to a complaint that appears to state little, if any, plausible claim for relief."

(Reporting by Andrew Chung in New York; Editing by Will Dunham)