WASHINGTON — The U.S. disrupted an undisclosed number of terrorist plots tied to the Independence Day holiday, highlighting the risk of an attack on American soil inspired by Islamic State extremists, FBI Director James Comey said.

Comey said Thursday that the Federal Bureau of Investigation had arrested at least 10 people in the past four weeks on suspicion they were linked to Islamic State. Some of those detained had plans involving the holiday weekend, he said, without providing further details.

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

Comey’s remarks came a day after he warned U.S. lawmakers about the growing threat posed by terrorists and other criminals who communicate over encrypted applications that can’t be deciphered. Dozens of Americans were communicating with Islamic State over such secretive networks, Comey told reporters Thursday at FBI headquarters, saying that potential terrorists are “going dark” by using such tools.

FBI officials have said that American sympathizers often follow Islamic State militants on Twitter and send them direct messages through the social media platform. Militants next direct the Americans to use secure communication tools that can’t be accessed by federal agents. Islamic State has more than 21,000 English-speaking followers on Twitter, federal authorities say.

Law enforcement officials have expressed growing alarm that Islamic State is successfully using social media to inspire homegrown extremists in the U.S. and Europe to act on their own, a departure from how other terrorist groups, such as al-Qaida, have long operated.

Comey said he and other FBI officials have met with executives of technology companies to find ways that would allow federal agents, armed with court orders, to access encrypted communications.

Those discussions have gone well, he said, and leaders of the companies understand the threat posed by Islamic State, which has seized large swaths of Syria and Iraq. The director praised Twitter Inc., in particular, for the way its executives have helped with terrorism and criminal investigations.

The Justice Department isn’t yet pushing for legislation that would require companies to allow federal agents armed with court orders access to communications. Instead, it is seeking to work with companies to devise a compromise.

Companies such as Apple and Google are using harder-to-crack encryption in smartphones to protect user data and privacy. Such tools, which help block hackers, are impeding criminal and law enforcement investigations into terrorists, Comey told the Senate Judiciary Committee on Wednesday.

Technology companies are working to block federal efforts to deliberately weaken encryption — a method of scrambling data so it’s only accessible with a special key — that protects e- mails, social media posts, instant messages and text messages.

They say revelations of government spying in documents leaked by former U.S. contractor Edward Snowden have eroded consumer confidence in the security of their products and will cost more than $35 billion in lost sales and contracts by 2016, according to a report by the Information Technology and Innovation Foundation.