The number of Americans traveling to the Middle East to fight alongside Islamic State has dropped, but the terrorist group’s efforts to radicalize people online is getting a major boost from encryption technology, FBI Director James Comey said Wednesday.

Since August, just one American a month has traveled or attempted to travel to the Middle East to join the group, compared with around six to 10 a month in the preceding year and a half, Mr. Comey told reporters in a round table meeting at FBI headquarters.

However, federal authorities have their hands full trying to counter Islamic State’s social media appeal. Of around 1,000 open FBI investigations into people who may have been radicalized across the U.S., about 80% are related to Islamic State, Mr. Comey said.

The increasing use of encrypted communications is complicating law enforcement’s efforts to protect national security, said Mr. Comey, calling the technology a “huge feature of terrorist tradecraft.”

The FBI director cited Facebook Inc.’s WhatsApp texting service, which last month launched end-to-end encryption in which only the sender and receiver are able to read the contents of messages.