FBI Director James Comey said Thursday that the agency believes it stopped potential acts of violence in the month before the July 4 holiday.

Comey said authorities suspect that some of the more than 10 people arrested during that time were planning to commit 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 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.

"I do believe that our work disrupted efforts to kill people, likely in connection with July 4," Comey said.

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. A law enforcement bulletin issued ahead of the holiday, Comey said, was motivated by the overall threat.

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, or when they might act. 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, noting 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, of a future plot to kill a conservative blogger known for provoking Muslims but decided to change plans.

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 investigative methods.