WASHINGTON — Fewer Americans are traveling to fight alongside the Islamic State and the power of the extremist group’s brand has significantly diminished in the United States, FBI Director James Comey said Wednesday.

The FBI encountered “six, eight, 10” Americans a month in 2014 and the first half of 2015 who traveled to the Middle East or tried to go there to join the Islamic State, but that number has averaged about one a month since last summer, Comey said.

“There’s no doubt that something has happened that is lasting, in terms of the attractiveness of the nightmare which is the Islamic State to people from the United States,” he told reporters during a wide-ranging round-table discussion Wednesday.

He did not offer an explanation for the decline, though the FBI has worked aggressively in the last year to identify and intercept Americans who might be determined to reach Syria. One other possibility is that the Islamic State has encouraged more of its followers to carry out violence at home, and Comey acknowledged Wednesday that the group’s ability to encourage and inspire “troubled souls” remains a concern.

The FBI still has “north of 1,000” cases in which agents are trying to evaluate a subject’s level of radicalization and potential for violence.

“There’s still a presence online, and troubled people are still turning to this and at least being interested in it,” Comey said. “But they’ve lost their ability to attract people to their caliphate from the United States in a material way.”

After a review of the evidence, the FBI has concluded that the San Bernardino, California attack that killed 14 people in December was inspired by the Islamic State, he said.