Some American officials and national security experts said the intense focus on the Islamic State had distorted the picture of the terrorism threat that has emerged from the chaos of Syria’s civil war, and that the more immediate threats still come from traditional terror groups like Khorasan and the Nusra Front, which is Al Qaeda’s designated affiliate in Syria…

It is difficult to assess the seriousness and scope of any terror plots that Khorasan, the Nusra Front or other groups in Syria might be planning. In several instances in the past year, Nusra and the Islamic State have used Americans who have joined their ranks to carry out attacks — including at least one suicide bombing — inside Syria rather than returning them to strike inside the United States.

Beyond the militant groups fighting for control of territory, Syria has become a magnet for Islamic extremists from other nations who have used parts of the country as a sanctuary to plot attacks.

“What you have is a growing body of extremists from around the world who are coming in and taking advantage of the ungoverned areas and creating informal ad hoc groups that are not directly aligned with ISIS or Nusra,” a former senior law enforcement official said.