Approximately 300 American or U.S.-based ISIS sympathizers have been identified as active online recruiters for the terror group, according to a study released Tuesday.

Researchers at George Washington University's Program on Extremism said that the recruiters primarily use Twitter to encourage like-minded individuals to travel to Iraq or Syria to join the self-proclaimed ISIS "caliphate", or to plan attacks within the U.S.

The study said that while the number of ISIS supporters is not as large in the U.S. as in other Western countries, "ISIS-related mobilization in the United States has been unprecedented." Citing American authorities, it said approximately 250 Americans had traveled or attempted to travel to Iraq or Syria, while 900 active investigations were in progress against ISIS sympathizers in all 50 states.

In one case, the study's authors said, "[a] seemingly naive individual posted general questions about religion [on Twitter], to which ISIS supporters quickly responded in a calm and authoritative manner. After a few weeks, the accounts of hardened ISIS supporters slowly introduced increasingly ardent views into the conversation."

"The new recruit was then invited to continue the conversion privately," the authors continued, "often via Twitter's Direct Message feature or on other private messaging platforms."

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The study, titled "ISIS in America: From Retweets to Raqqa", also found that 71 people had been charged in the U.S. with ISIS-related activities since March 2014. 56 of those arrests have occurred since the start of 2015, the most terrorism-related arrests in any year since the Sept. 11, 2001 attacks.

In addition to online recruiters, the study also identifies several small groups of friends or acquaintances who support ISIS. One of the cells, the researchers said, is based in Texas and "revolves around a few charismatic individuals and an Islamic studies group." The second is located "in the suburbs of a large Midwestern city [and] appears to be composed of former high-school friends and a handful of their acquaintances."

The study's authors did not identify the groups further, saying they did not want to compromise potential criminal investigations.

The study finds that defendants accused of ISIS-supporting activities have an average age of 26 and are overwhelmingly male. Just over half had traveled or attempted to travel abroad, and approximately one quarter were involved in plots to attack the U.S.

"The profiles ... differ widely in race, age, social class, education, and family background," the study's executive summary said. "Their motivations are equally diverse and defy easy analysis."

The summary added that U.S.-based sympathizers' actual involvement with ISIS ranged from "those who are merely inspired by its message to those few who reached mid-level leadership positions within the group."