Active online supporters of the Syria-based Islamic State movement now include about 300 people identifiable as Americans, said a study by academic experts published on Tuesday.

George Washington University's Program on Extremism said in the study that the number of other Americans who passively "consume" Islamic State propaganda runs to "several thousand," though they are not necessarily active supporters of the group.

Twitter is the "platform of choice" most widely used by the active core of American supporters of Islamic State, it said.

American ISIS activists and sympathizers also use other social media, ranging from open forums such as Facebook, Google+ and Tumblr to more secretive messaging apps including Kik, Telegram, surespot and the dark web, the study said.

American online ISIS supporters are sufficiently active and noisy to have established themselves as "nodes," or leading voices promoting Islamic State themes, while others serve as "amplifiers," who repost materials from more prominent activists.

The study said U.S. Islamic State activists have helped craft a "unique innovation" in militant messaging. This involves creation of "shout out" accounts, which enable activists to "introduce new pro-ISIS accounts to the community and promote newly created accounts of previously suspended users, allowing them to quickly regain their pre-suspension status."

The study noted that, although American social media accounts linked to ISIS are regularly suspended, among the activists such suspensions have become a "badge of honor and a means by which an aspirant can bolster his or her legitimacy."

Operators of suspended accounts usually set up and start using a newly created account, using only a variation of the previous user name, "within hours" of most suspensions, it said.

Even though Islamic State's forerunners first emerged after the U.S. invasion of Iraq in 2003, Islamic State's "message did not exist" before the organization established itself in Syria and became involved in a civil war against the government of President Bashar al Assad, said Lorenzo Vidino, one of the study's principal authors.

Vidino cited FBI statistics indicating that U.S. authorities are currently pursuing 900 terrorism-related investigations in all 50 U.S. states. His study noted that 71 individuals have been charged by U.S. authorities with ISIS-related offenses since March 2014, with 51 of those arrests occurring in 2015.