U.S. prosecutors have charged more than 80 people with terror-related offenses since the start of 2014 -- at least 60 alone this year, and the lion's share of the cases are related to ISIS, according to the Justice Department.
President Barack Obama recently chided the media for seeding Americans' anxiety by focusing excessively on ISIS. But the pace of prosecutions and investigations by his Justice Department underscore the cause for concern.
The prosecutions are widely dispersed across the U.S., in 30 different federal court districts. The ages of those charged varies; just over half are under 25 years old and one third are under 21.
"In some ways, it's an unprecedented threat environment that we're facing," John Carlin, assistant attorney general for national security, told CNN.
Since ISIS burst on the scene by seizing swaths of territory in Iraq and Syria in 2014, the jihadist group has rewritten the strategy used by other groups that preceded it, including al Qaeda.
The group has innovated the use of social media to encourage westerners to either travel to become a fighter with the group, or to encourage attacks at home.
The federal cases range from attempting to travel to join the terrorist group to those who allegedly plotted attacks.
This year also saw the first case involving cyberterrorism: a Kosovo national living in Malaysia is awaiting extradition to U.S. for allegedly using cyberattacks to steal personal information of U.S. military members and passing it to an ISIS hacking group, which in turn posted it online to encourage attacks on the military members.
In another first, an ISIS recruit in Maryland received thousands of dollars from ISIS operatives overseas to fund plots here in the U.S.
As a result, ISIS now is the top national security problem for U.S. authorities. And that's reflected in the more than 900 FBI investigations ongoing across 50 states, the vast majority of which are related to ISIS, U.S. officials say.
Carlin notes that the U.S. still is on guard for more-organized plots of the type al Qaeda has tried to carry out against aviation and against U.S. interests overseas, or in the U.S.
"We still face a threat from those who would like to carry out large scale attacks," Carlin says. "At the same time we have seen the adoption of a new strategy where they are trying to crowd-source terrorism."
The challenge for U.S. authorities is that ISIS has managed to appeal to a wide demographic of Americans, and radicalization in private can be hard to spot.
The San Bernardino, California, terrorist attack, carried out by an American citizen and his wife who previously hadn't raised any law enforcement concern illustrates the problem, officials say.
Part of the solution for U.S. officials has been to arrest suspects even if there's not enough proof of terrorist plots. For instance, in the months following a foiled attack on a Prophet Mohammed drawing contest in Garland, Texas, the FBI increased round-the-clock surveillance of suspected jihadist sympathizers. More than two dozen arrests followed, including some for charges like lying to the FBI.
FBI Director James Comey has said in recent weeks that some of the arrests were done as a way to get suspects off the streets, because of concerns the agency wouldn't be able to tell if someone has decided to initiate an attack.
In the wake of San Bernardino, some U.S. officials say concerns about similar threats may prompt another spate of investigations and prosecutions.