A 17-year-old high-school student from Northern Virginia was arrested by the FBI for allegedly helping a man travel to Syria to join the Islamic State group, The Washington Post reported, citing officials familiar with the matter. Although the boy was charged as a juvenile, prosecutors are reportedly trying to move the case to an adult court.

An FBI official, whose name was not revealed by the Post, reportedly said that investigators had conducted surveillance outside the teen’s home for over a month before taking him into custody. The boy, a student at Prince William County’s Osbourn Park High School, is accused of disseminating ISIS propaganda online and helping a man to travel to Syria. The name of this person, who has allegedly already joined ISIS, was not revealed.

Dustin O’Bryant, editor of the website Coin Brief, which had employed the teenager as a freelancer, said that he was “extremely, extremely surprised” when he heard of his arrest, according to a CNN report.

“He did not come across as radical in any way,” O'Bryant reportedly said. “I hope there's some sort of misunderstanding here, and that he didn't know what he was doing.”

The teenager’s arrest came just hours after Adam Dandach, 21, a California resident detained in July for attempting to join ISIS in Syria, was indicted in a Santa Ana federal court.  Last week, three men from the Brooklyn borough of New York City, were arrested and charged with conspiring to provide aid to the militant group.

According to a recent report by The International Centre for the Study of Radicalisation and Political Violence -- a London-based think tank -- the number of foreign fighters that have joined extremist groups, including ISIS, in Syria and Iraq has exceeded 20,000. Of these, nearly 100 are believed to be U.S. nationals.

“The estimated worldwide total is 20,730. This makes the conflict in Syria and Iraq the largest mobilization of foreigner fighters in Muslim majority countries since 1945. It now surpasses the Afghanistan conflict in the 1980s, which is thought to have attracted up to 20,000 foreigners,” the organization said, in its report.