Carolyn Kaster, Associated Press
In this July 8, 2015, photo, FBI Director James Comey testifies during the Senate Judiciary Committee hearing on Capitol Hill in Washington. The FBI stopped several potential acts of violence in the month before the July 4 weekend, Comey said on July 9.

WASHINGTON — The FBI stopped several potential acts of violence in the month before the July 4 weekend, FBI Director James Comey said Thursday.

Comey said he believes that some of the more than 10 people arrested during that time were planning acts of violence tied to the holiday. But he declined during a wide-ranging discussion with reporters to describe any of the potential plots that might have been thwarted or to identify specific individuals the FBI thought might carry out at an attack.

Federal agents had ramped up efforts in recent months to arrest Islamic State sympathizers across the country, arresting more than 10 in the last four weeks in places including New Jersey, Ohio and North Carolina.

The FBI and Department of Homeland Security had warned of a heightened terror threat tied to the July 4 weekend, but had not publicly identified any specific plot they were tracking.

Comey also said the current crop of Westerners attracted by the Islamic State's messaging is so unpredictable that it can be hard for federal authorities to ever be sure of their plans. Whereas Al-Qaida would train operatives and carefully scope out targets, the concern among law enforcement officials is that the Islamic State is motivating people to commit violence "on a very short string."

"We face people who are highly unpredictable," Comey said. "We cannot count on the fact that they'll be looking to do something on July fourth, so July third, we really got to lock these guys up."

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. Authorities say he had been scheming with other men, both now facing charges, of plotting to kill a conservative blogger known for provoking Muslims.

Comey said that some of the people who had been arrested in the last month had been communicating on encrypted platforms — a concern he discussed before Congress on Wednesday — but he acknowledged that, in these particular cases, agents were able to use other means.

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