Private Superforecasting Workshop asks:
Will there be a complex coordinated terrorist attack (CCTA) in the United States before 1 September 2021?
CCTAs are a major concern for law enforcement and emergency planners (FEMA). For the purposes of this question, a CCTA is an  act of terrorism  involving multiple perpetrators working in concert  initiated with little or no warning  and employing one or more weapon systems (e.g., firearms, explosives, fire as a weapon, poison gas)  that is intended to result in large numbers of casualties. For the purposes of this question, "terrorism" is as defined by federal law in 18 U.S.C. § 2331(5) (Cornell). Examples of CCTAs include the 2004 Madrid train bombings, the 2008 Mumbai attacks, 2013 Boston Marathon bombings, and the 2015 San Bernardino, CA, office attack.