George Washington University’s Program on Extremism released a study Monday focused on Americans who traveled to Iraq or Syria to join the Islamic State (ISIS).

The study, titled “American Jihadists in Syria and Iraq,” provides details on 64 of the 300 or so jihadists the Federal Bureau of Investigation (FBI) identified as those who attempted to or did travel abroad to join the radical Islamic group. The 64 on whom the study focused made it to the Middle East, and 12 of those returned to the United States.

Of those twelve, nine were arrested, one returned to Syria and died in a suicide attack, and two, while known by authorities, remain free. One, according to the study, has planned an attack inside the U.S.

“As of January 1, 2018, no returned travelers from Syria and Iraq have successfully committed a terrorist attack in the U.S. following their re-entry,” the study states.

“One individual — Abdirahman Sheik Mohamud — who returned to the U.S. from fighting in Syria and Iraq was directly instructed by his group’s leadership to carry out an attack in the U.S.,” the study states. “However, he was apprehended in the early planning stages of the attack after re-entry.”

The study notes traveling abroad to join the Islamic State is “the most popular” way individuals choose to support the terrorists:

Travel or attempted travel to jihadist-held territory in Syria and Iraq is one of the most popular forms of mobilization for American jihadist sympathizers. In addition to successful travelers, at least 50 Americans attempt- ed to travel but were prevented from doing so by law enforcement. These cases constitute approximately one- third of the 153 Americans who have been arrested on Islamic State–related charges from 2011 to 2017. Cases of travel or attempted travel to Syria or Iraq have steadily decreased since 2015.

National Public Radio (NPR) played up the fact that none of the 12 returnee jihadists have carried out an attack on U.S. soil. Its reporting included an interview with one of the study’s authors.

“I think what we were struck with was the few numbers of returnees that we saw,” Seamus Hughes told NPR. “There was always concern that this wave of what the FBI would call ‘the terrorist diaspora’ would come back.”

“In many ways, it’s just a trickle right now,” Hughes said.

Hughes also talked to NPR about one of the American jihadists who took on a leadership role in the Islamic State, Zulfi Hoxha, who is of Albanian descent and lived in New Jersey.

“Hoxha traveled to Syria in 2015, and U.S. authorities have described him as a ‘senior ISIS commander,'” NPR reported. “He appeared in two ISIS propaganda videos, including one where he beheads a prisoner, according to Hughes.”

However, there are many of what the study calls “travelers,” who remain at large.

“Authorities are understandably concerned about the uncertain number of travelers who remain at large,” the study states. “This contingent represents a serious threat, as some of them may act as nodes for American jihadist recruitment networks.”

“As jihadist organizations face increased pressure in Syria and Iraq, several of them may also flee the region and join other affiliates or associated groups,” the study states.

Other statistics included in the study: