By Daniel White
Updated: December 1, 2015 4:45 PM ET

Authorities in the U.S. have arrested more than 50 people so far this year for activities related to the Islamic State of Iraq and Greater Syria (ISIS), according to a new study released Tuesday.

So far, 2015 has already seen the largest number of terrorism-related arrests in a single year since 9/11, according to an analysis of legal documents and social media accounts by George Washington University. Many of those arrests were connected to ISIS—of the 71 people who have been charged with supporting or plotting with ISIS since March 2014, 56 of those arrests came in 2015 alone, the study found.

According to the study, “ISIS in America: From Retweets to Raqqa,” there are hundreds of American ISIS sympathizers who have not been arrested, and they are “particularly active on Twitter,” creating accounts that get suspended in a cat-and-mouse game with authorities. Most ISIS supporters in the U.S. are radicalized online, but many also discuss their interest in ISIS with others in person, the study found.

While the threat of radicalization exists in the U.S., the study concludes that it is “significantly smaller, more decentralized, and less professional than that of most European countries.” Authorities in U.K. alone arrested nearly 300 people on terrorism-related charges in the past year, according to the Wall Street Journal.

Authorities have spoken to some 250 Americans regarding their attempts to travel to Syria or Iraq to join ISIS, while authorities have 900 active investigations against ISIS sympathizers, the George Washington study found.

The six-month study’s release comes shortly after the Nov. 13 terrorist attacks in Paris, claimed by ISIS, which left 130 people dead.

Contact us at editors@time.com.

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