He would not discuss the number or nature of the plots, but earlier Thursday, U.S. officials told CNN that the thwarted plots included targets "coast to coast," and in fitting with calls by ISIS to attack in any way possible, they were unsophisticated, relying on guns, knives and other weapons.
Also fitting with recent patterns, investigators believe the plots, though not directed by overseas terror groups providing specific means of attack or specific targets, were "enabled" by actors abroad, including recruiting the suspects and encouraging them to carry out attacks on U.S. soil.
No further details were immediately available about how the plots were thwarted.
Speaking to reporters at the J. Edgar Hoover Building in Washington, Comey said the FBI is seeing a "mix" of ISIS fighters directing and inspiring people in the U.S. to launch an attack.
"Sometimes there is a suggestion from an ISIS fighter to do something specific" such as target a certain location, he said.
Comey called this evolution in the current terrorism threat "crowdsourcing terrorism," and he said it typically begins over social media before the conversation "goes dark."
"Dozens" of people in the U.S. are communicating with ISIS terrorists overseas through encryption, he added.
He explained the process as beginning with someone who follows an ISIS fighter on Twitter, and then the conversation moves to direct messaging where information is exchanged to move the conversation, to a more secure app where it then "disappears."
In the last four weeks, the FBI has arrested more than 10 people who are tied to ISIS, and Comey said some of them were communicating with ISIS through encryption. Comey said ISIS's influence online is "living proof that social media works" and that what the U.S. is doing to degrade ISIS online is "clearly not enough."
In testimony before a Senate panel on Wednesday
, Comey said ISIS has 21,000 English-language followers on Twitter now, turning that in to a fertile recruiting tool.
With this use of social media, Comey said, recruits have a "devil in their pocket all day long that says 'Kill, kill, kill.'"
The news about the plots is significant because of the increased security measures federal, state and local officials took in the days leading up to the holiday. The FBI, Department of Homeland Security and National Counterterrorism Center outlined in a memo several weeks ago
that listed the holiday weekend and several upcoming Prophet Mohammed drawing events as possible targets for attacks.
A CNN analysis of Justice Department records and court documents last month found that at least 49 alleged ISIS supporters had been charged by U.S. authorities since the start of 2015.
That number did not include Elton Simpson and Nadir Soofi
, who died during an attack at a Mohammed cartoon contest in Texas in May, or Usaamah Rahim
, who was killed in Massachusetts last month during a federal terror investigation.
In the Texas cartoon attack, neither Simpson or Soofi made it into the city's Curtis Culwell Center after being shot dead by an overmatched Garland police traffic officer who was part of the on-site security contingent. That officer was wounded but survived. An Arizona man was indicted last month
for allegedly providing guns to, shooting with and talking through the plan to carry out the attack.
Rahim was fatally shot by police after he allegedly wielded a military knife at counterterrorism officers. The FBI said tapped phone conversations support its claim that Rahim was going to carry out a knife attack against police at any moment
, which is why the joint anti-terror team of federal and local officers confronted him when it did.