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TWITTER, INC. v. TAAMNEH 

Syllabus 

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. 6–31. 

(a) In 2016, Congress enacted the Justice Against Sponsors of Ter-
rorism 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
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. 6–8. 

(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. 8–21. 

(1) Nothing in the statute defines any of the critical terms in the 
phrase  “aids  and  abets,  by  knowingly  providing  substantial  assis-
tance.”  The term “aids and abets,” however, is a familiar common-law 
term  and  thus  presumably  “brings  the  old  soil”  with  it.  Sekhar  v. 
United States, 570 U. S. 729, 733.  Congress also provided additional 
context in JASTA by pointing to Halberstam v. Welch, 705 F. 2d 472, 
as “provid[ing] the proper legal framework” for “civil aiding and abet-
ting  and  conspiracy  liability.”  130  Stat.  852.  Halberstam’s  legal 
framework, viewed in context of the common-law tradition from which 
it arose, confirms that “aids and abets” in §2333(d)(2) refers to a con-
scious, voluntary, and culpable participation in another’s wrongdoing.  
Pp. 9–17.

(i) In Halberstam, the D. C. Circuit undertook an extensive sur-
vey of the common law with respect to aiding and abetting and synthe-
sized the surveyed cases as resting on three main elements: (1) there 
must  be  a  wrongful  act  causing  an  injury  performed  by  the  person
whom  the  defendant  aided;  (2)  at  the  time  assistance  was  provided, 
the defendant must have been “generally aware of his role as part of
an overall illegal or tortious activity;” and (3) the defendant must have 
“knowingly and substantially assist[ed] the principal violation.”  705 
F. 2d, at 477.  The court then articulated six factors to help determine
whether a defendant’s assistance was “substantial.”  They are (1) “the 
nature of the act assisted,” (2) the “amount of assistance” provided, (3)
whether the defendant was “present at the time” of the principal tort, 
(4) the defendant’s “relation to the tortious actor,” (5) the “defendant’s
state of mind,” and (6) the “duration of the assistance” given.  Id., at 
488 (emphasis deleted).  Halberstam also clarified that those who aid 
and abet “a tortious act may be liable” not only for the act itself but
also “for other reasonably foreseeable acts done in connection with it.” 
Id., at 484.  Finally, the court warned that its formulations should “not