WASHINGTON — At least three dozen people in the United States suspected of ties to the Islamic State were under heavy electronic or physical surveillance even before the Paris attacks, senior American officials say. But unlike the attackers in France, the officials say, the majority of those under investigation here never traveled to Syria to fight alongside the Islamic State or receive training from it.

In many ways, the officials say, that makes the American investigations even harder. Those under investigation typically have little terrorism expertise or support from a cell, which makes thwarting an Islamic State-inspired attack in the United States less like stopping a traditional terrorist plot and more like trying to prevent a school shooting.

Read the article “U.S. Investigators Struggle to Track Homegrown Islamic State Suspects” on nytimes.com.