The FBI calls them “homegrown violent extremists” — Americans who likely have never left the country, but have self-radicalized and aligned themselves with foreign terrorists groups. They can often plan violent attacks in hours or days, targeting people in their communities.

This emerging group is now the government’s primary concern in combating terrorism, FBI Director Christopher Wray told a gathering of hundreds of Utah law enforcement officers at the state’s National Security and Anti-Terrorism Conference in Salt Lake City on Wednesday.

Wray said the FBI is still concerned with foreign threats from groups like al-Qaida and the Islamic State terrorist group, also known as ISIS, but noted a growing number of cases where people in the United States have connected with terrorists online and adopted their ideals before carrying out acts of violence.

The homegrown violent extremists — or HVEs — are more difficult to track than traditional terrorist groups, Wray said, because there is not a typical profile of the type of person who gravitates toward terroristic behavior.

“We’re seeing a worrisome trend where people are being radicalized at younger and younger ages,” he said.

He pointed to a case in Utah as an example, where a teen boy is accused of removing the American flag at Hurricane High School in February and replacing it with a homemade Islamic State flag. Weeks later, the teen allegedly brought a backpack with a makeshift explosive device in it to a Washington County high school. The device did not go off, and no one was injured. The teen, who now faces charges in adult court, has pleaded not guilty.

According to charging records, the boy told police that if the bomb had gone off, he would have “laid low” for a while before trying to hang another ISIS flag somewhere in the community to “make it look like ISIS is here.”

“Then maybe after that try to contact ISIS,” the teen told police, “but I don’t really know how to do that. I need to do more research on that.”

The FBI director said Wednesday that in cases like this, there often are telltale signs that a person is becoming radicalized. He urged local police agencies to continue to strengthen their partnerships with community members, especially those working in the mental health field.

“Most HVEs are not entirely unknown,” Wray said. “There is almost invariably a family member or a friend, a neighbor, a co-worker or someone in the community who saw the radicalization happening in real time. We need those people to speak up.”