Volume 598 U. S. Part 2 Pages 471–507

OFFICIAL REPORTS OF

THE SUPREME COURT May 18, 2023

Page Proof Pending Publication

REBECCA A. WOMELDORF reporter of decisions

NOTICE: This preliminary print is subject to formal revision before the bound volume is published. Users are requested to notify the Reporter of Decisions, Supreme Court of the United States, Washington, D.C. 20543, pio@supremecourt.gov, of any typographical or other formal errors. OCTOBER TERM, 2022 471

Syllabus

TWITTER, INC. v. TAAMNEH et al. certiorari to the united states court of appeals for the ninth circuit No. 21–1496. Argued February 22, 2023—Decided May 18, 2023 In 2017, Abdulkadir Masharipov carried out a terrorist attack on the Reina nightclub in Istanbul, Turkey, on behalf of the Islamic State of Iraq and Syria (ISIS), a designated Foreign Terrorist Organization. Masharipov killed Nawras Alassaf and 38 others. Alassaf 's family then brought this suit under 18 U. S. C. § 2333, an Antiterrorism Act (ATA) provision that permits U. S. nationals who have been “injured . . . by reason of an act of international terrorism” to fle a civil suit for damages. Instead of suing ISIS directly under § 2333(a), the plaintiffs (respondents here) invoked § 2333(d)(2) to sue three of the largest social-media companies in the world—Facebook, Twitter (petitioner here), and Google (which owns YouTube)—for aiding and abetting ISIS. The parties today agree on the basic aspects of these platforms: Bil- lions of people from around the world have signed up for them and Page Proof Pending Publication upload massive amounts of content each day. Defendants proft from that content by placing advertisements on or near it and use “recom- mendation” algorithms that match content, advertisements, and users based on information about the use, advertisement, and content being viewed. As the parties represent things, the algorithms here match any content with any user who is more likely to view that content, and the platforms perform little to no front-end screening on any content before it is uploaded. Plaintiffs, however, allege that for several years the companies have knowingly allowed ISIS and its supporters to use their platforms and “recommendation” algorithms as tools for recruiting, fundraising, and spreading propaganda; plaintiffs further allege that these companies have, in the process, profted from the advertisements placed on ISIS' tweets, posts, and videos. The District Court dismissed the complaint for failure to state a claim, but the Ninth Circuit reversed. Held: Plaintiffs' allegations that these social-media companies aided and abetted ISIS in its terrorist attack on the Reina nightclub fail to state a claim under 18 U. S. C. § 2333(d)(2). Pp. 482–507. (a) In 2016, Congress enacted the Justice Against Sponsors of Terror- ism Act (JASTA) to impose secondary civil liability on anyone “who aids and abets, by knowingly providing substantial assistance, or who conspires with the person who committed such an act of international 472 TWITTER, INC. v. TAAMNEH

Syllabus

terrorism.” § 2333(d)(2). The question here is whether the conduct of the social-media company defendants gives rise to aiding-and-abetting liability for the Reina nightclub attack. Pp. 482–484. (b) The text of JASTA begs two questions: What does it mean to “aid and abet”? And, what precisely must the defendant have “aided and abetted”? Pp. 484–497. (1) Nothing in the statute defnes any of the critical terms in the phrase “aids and abets, by knowingly providing substantial assistance.” Th