The head of the U.S. Federal Bureau of Investigation said Thursday that agents broke up a number of terror plots in the weeks leading up to the Independence Day holiday.
"I do believe our work disrupted efforts to kill people, likely in connection with July 4," Director James Comey told reporters.
He said more than 10 people were arrested. But he gave no information about what they were plotting or where they planned to strike.
Separately, a national security source said multiple overseas plots by Islamic State sympathizers had also been halted in recent days.
The FBI and the Department of Homeland Security had issued warnings of possible terrorist attacks to be carried out on July 4, when millions of Americans were celebrating Independence Day with concerts, parties, picnics and fireworks shows.
Comey said the Islamic State group was encouraging would-be homegrown terrorists to act and that some of those arrested had been communicating with the militants through encrypted message platforms.
The FBI has pressured tech companies to remove encryption that gives users privacy protections that cannot be broken by law enforcement. Comey estimated that dozens of people influenced by Islamic State have "gone dark" and disappeared from the FBI's watch because of encrypted data.
The director said said the FBI was especially challenged because Islamic State-inspired terrorists are highly unpredictable and act on the spur of the moment.
Comey noted that the FBI does not discount the possibility that an individual who plans an act of terrorism for a particular day such as July 4 might randomly decide to kill someone earlier.
He cited as an example the case of a 26-year-old terror suspect who was fatally shot by police in Boston last month after authorities said he lunged at them with a military-style knife. The FBI has said he had been scheming with other men, both now facing charges, on a future plot to kill a conservative blogger known for provoking Muslims but decided to change plans.
Some information for this report came from AP and Reuters.