Army Col. Steve Warren, spokesman for the U.S.-led coalition against the Islamic State (ISIS), said Sunday that jihadists appear to be deserting the group in growing numbers, leaving ISIS strongholds badly secured but prompting new concerns that defecting jihadists will return home battle-hardened and ready to execute “lone wolf” attacks.

Iraq appears particularly vulnerable to jihadists fleeing the group, Col. Warren said, as coalition soldiers find evidence of “wholesale defections, sparsely manned checkpoints and elite foreign fighters pressed into mundane duty.” Col. Warren noted at least one instance of 90 ISIS terrorists handing their weapons over to Kurdish Peshmerga soldiers in Kirkuk, about an hour’s drive away from the capital of Iraqi Kurdistan, Erbil. Warren’s report also notes that terrorists known to American intelligence as high-ranking, well-trained jihadists have been spotted manning checkpoints, described by USA Today as “mundane duty” that would go to a lower-ranking jihadi. This indicates the number of eligible fighters for such jobs is dwindling.

The American coalition also announced it believes about 23,000 ISIS terrorists have been killed since the campaign began but that the group counts on up to 30,000 jihadists in both Iraq and Syria, most protecting strongholds like Raqqa, the group’s “capital” in Syria, and Mosul, the second-largest city in Iraq.

The coalition report’s indication that the Islamic State may be facing a slower influx of recruits follows an August report from the German government, finding the number of German nationals traveling to Syria to join ISIS has also dwindled. Officials noted that they expect the number of people leaving Germany to join the Islamic State “will continue to increase, probably not as dramatically as we experienced last year, but ISIS, Syria and Iraq and this battlefield are still attractive for young people from Germany who want to become jihadists.”

Officials have not speculated on what is driving would-be jihadists to increasingly go home or stay home. As the Islamic State has risen in international prominence, so have stories of the horrors facing recruits in Iraq and Syria. As early as December 2014, young men who traveled to the Middle East began complaining in anonymous messages that charging their iPods in the middle of jihad was a struggle worth heading home for. Those who managed to escape have claimed to be more comfortable in their home nations’ jails than “free” among jihadis. Others found the experience of jihad so off-putting, they converted to Christianity following their flight.

In October, photos began circulating on social media of large piles of facial hair and razors in villages where ISIS jihadists had decided to flee, shaving and wearing women’s garb to sneak back into Turkey and, ultimately, Europe.

Those who hope to flee and fail meet harrowing deaths by beheading or, in the case of one European teen girl who tried to escape, by being beaten to death.

In addition to the bad publicity joining the Islamic State has generated in the past year, the group itself has taken to encouraging jihadists not to come to Syria or Iraq, and instead plan attacks in their home countries. Reports indicate that ISIS leaders are particularly interested in recruiting “lone wolves” in Brazil in order to stage a terrorist attack during the 2016 summer Olympics.

Islamic State videos have also directly called for European citizens to commit acts of terror in their homelands. “My brothers and sisters, either you come here and join the Mujahideen, or you lead the jihad in Germany and Austria. You do not need much, just take a large knife and kill every unbeliever. They are like dogs!” says one terrorist in a video released in August. Three months later, ISIS-linked terrorists staged multiple attacks in Paris believed to have been planned in Syria by some who returned to France posing as refugees.

Returning jihadists pose as much of a threat as lone wolves to the United States, experts fear. The Islamic State has claimed only one attack in the United States—the shooting outside an art installation in Garland, Texas, this summer—orchestrated by American citizens who had never traveled to Syria or Iraq for terror training. The men had tweeted their allegiance to Islamic State “caliph” Abu Bakr al-Baghdadi before the shooting, however, allowing the group to claim responsibility.

Unlike those terrorists, however, officials estimate that up to 50 Islamic State recruits have traveled to ISIS-controlled territory and returned to American soil. According to Rep. Michael McCaul (R-TX), who chairs the House Homeland Security Committee, “hundreds” of American nationals have traveled to Iraq and Syria. “50 have come back to the United States,” he claimed on Sunday. “We don’t want these foreign fighters coming into the United States from visa waiver countries,” he explained, adding that the U.S. has had, “in the homeland, 18 plots stopped that were ISIS-related. We’ve arrested 70 ISIS followers. And we have 1,000 investigations in all 50 states.”

The number is consistent with that reported by FBI Director James Comey in October, when he indicated that at least 900 investigations against suspected jihadists in the United States were underway.