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Between 2006 and 2009, fewer than 1 percent of mass-casualty events — intentional, violent attacks where four or more victims are killed within a 24-hour period — had a link to extremism. Between 2018 and 2021, more than 5 percent did, according to a FiveThirtyEight analysis of two databases from the University of Maryland’s National Consortium for the Study of Terrorism and Responses to Terrorism and a collaboration between USA Today, The Associated Press and Northeastern University.1 This data is supported by reports from the FBI, the Department of Homeland Security and the Anti-Defamation League, all of which show a marked increase in violent attacks linked to extremism in recent years.

Over the last decade and a half, the number of mass-casualty events each year has remained relatively flat. In 2006, for example, there were 38 mass-casualty events in the U.S., resulting in the deaths of 183 people, according to the USA Today/AP/Northeastern database. In 2021, there were 35 events, resulting in the deaths of 172 people; there were also an average of 31 mass-casualty events for each year from 2006 through 2021. Yet despite the total number of mass killings staying static, the number of events with extremist ties has increased, resulting in a higher percentage of extremist-linked mass killings.

There also has been a rise in the number of extremist-linked violent plots, according to the data from START. When extremists consider violent acts, they don’t always result in mass-casualty events. Sometimes perpetrators are caught by law enforcement before any violence can take place; other times fewer than four people are killed, even if the perpetrator likely intended to harm a greater number of people.

https://fivethirtyeight.com/features/extremism-mass-casualty-events-shootings