The Islamic State group's effort to inspire troubled Americans to violence has become more of a terror threat to the U.S. than an external attack by al Qaeda, the FBI director said Wednesday, July 22, according to the Associated Press.

FBI Director James Comey told an audience at the Aspen Security Forum that the Islamic State group, which has proclaimed a caliphate in parts of Syria and Iraq, has influenced a significant but unknown number of Americans through a year-long campaign on social media urging Muslims who can't travel to the Middle East to "kill where you are."

Twitter handles affiliated with the group have more than 21,000 English-language followers worldwide, he said, thousands of whom may be U.S. residents.

The FBI has arrested a significant number of people over the last eight weeks who had been radicalized, Comey said, without specifying a number. He repeated his previous disclosure, without elaborating, that several people were arrested who were planning attacks related to the July Fourth holiday. The bureau has hundreds of investigations pending into such cases across the country.

The U.S. has tracked dozens of Americans, ranging in age from 18 to 62, who have traveled to Syria or Iraq to fight with the Islamic State group, he said.

"I worry very much about what I can't see," Comey added, because he said Islamic State group recruiters use encrypted communication software to avoid U.S. eavesdropping.