Experts said the foreign fighter population is likely to grow significantly larger as the three-year-old conflict drags on.

“I don’t think 15,000 really scratches the surface yet,” said Andrew Liepman, a counterterrorism expert at Rand Corp. who formerly was the deputy director of the National Counterterrorism Center.
Since the start of the U.S.-led air campaign, analysts have sought to track whether the bombings would discourage would-be fighters or serve as a rallying cry for Islamists. Liepman said the steady numbers could mean that neither has occurred or, more likely, that both have happened to degrees that offset one another.

The air campaign “has probably discouraged some people and encouraged others,” Liepman said.