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. Comey spoke July 22 on the threat posed to the U.S. by the Islamic State Group at the Aspen Security Conference in Aspen, Colo.

ASPEN, Colorado — The FBI director says the Islamic State group's effort to inspire troubled Americans to kill at home has become more of a terror threat to the U.S. than an external attack by al-Qaida.

FBI Director James Comey told an audience at the Aspen Security Forum that the Islamic State group has seen success in recent months after a yearlong social media campaign to radicalize disaffected Americans and Europeans. He said the FBI had made a number of arrests over the last eight weeks of people who had been radicalized and has hundreds of investigations pending.

Comey's remarks signal a deepening concern about the impact of the Islamic State's effort to inspire terrorist violence. As recently as September, senior U.S. intelligence officials were downplaying the group's capacity to attack the U.S.