FBI Director James Comey said authorities have arrested "more than 10 people" over the past four weeks who have been radicalized through slick electronic recruitment efforts tied to the self-proclaimed Islamic state.
"We arrested them to try to thwart what they were up to," the FBI director said in a briefing with reporters Thursday in his Washington conference room.
"I do believe our work disrupted efforts to kill people, likely related to the Fourth of July," Comey added.
Not all of those arrested have been detained on terrorism charges, Comey said. And while the holiday has passed, he warned that new recruits who mobilize in response to social media contacts can be unpredictable.
"We have encountered a very tight flash to bang," meaning that a new batch of possible extremists no longer has the patience or training to lie in wait for months or years before a strike, as members of the core al-Qaida group once did.
Another senior FBI official said the bureau believes it likely did disrupt a plot or plots tied to last weekend's holiday, but there's still a lot more work to do to counteract online extremist propaganda.
Comey also revealed that his personal information had been compromised in the breach into the Office of Personnel Management. He shied away from citing specific numbers of people whose data had been exposed, but said it was in the "millions and millions ... a giga-boatload."
The Office of Personnel Management said Thursday afternoon that 21.5 million people were affected in the hacking, including 19.7 million, who applied for security clearances and another 1.8 million spouses or co-habitants of applicants.
"This is a very big deal from a national security perspective and a counterintelligence perspective," Comey said, because "it's a treasure trove of information about everyone who has worked for, does work for or tried to work for the United States."
He declined to say what, if anything, the Obama administration is doing to protect the identities of possible overseas intelligence assets, and whether the hackers have already attempted to take advantage of the information to exploit personal weaknesses. But he said those issues are the subject of ongoing high-level conversations and work inside the government.
As for reports this week that the U.S. government may still be open to a plea deal with NSA leaker Edward Snowden, Comey declined to share his personal views.
"My view is, he's a fugitive," Comey said. "I'd love to apprehend him, so he can enjoy the benefits of the freest and fairest justice system."