Estimating the Duration of Jihadi Terror Plots in the United States

The number of ongoing terror plots increases with the duration of time from plot initiation until plot execution or interdiction (whichever comes first), yet no estimate of the probability distribution governing terror plot duration has appeared in the open literature. To remedy this gap, jihadi terror plots in the United States were identified from terrorism-related indictments that occurred between 11 September 2001 and 30 June 2011 in addition to successful attacks. From a review of indictments, affidavits, and other publicly available information, it was possible to identify lower and upper bounds for the initiation date of the plot in question as well as the ending date corresponding to either the arrest of the suspect(s) eventually convicted or an attempted/actual act of terror. These observations enable estimation of the duration distribution of jihadi terror plots in the United States accounting for the censoring and truncation effects inherent with these data (technical details appear in the Appendix). The estimated mean plot duration equals 270 days (standard error of mean 40 days), while 95 percent of all plots are estimated to fall between 33 and 750 days. These estimates suggest that on average, approximately three ongoing terror plots have been active in the United States at any point since 11 September 2001.