America was comforted to learn in late July that an American-born fundamentalist who fought alongside Islamic militants inside Syria before executing a suicide attack on Bashar al-Assad’s troops in May had freely traveled from Syria to the United States.

“This incident exemplifies the challenges faced by the FBI in detecting US citizens who seek to travel to Syria to engage in jihad,” an FBI spokesman said of the benign infiltration of U.S.-born Moner Mohammad Abusalha. Apparently, that challenge has not yet been overcome by American defense officials.

On Monday, an Obama administration official confirmed that some American citizens who fought with ISIS in Iraq and Syria have come back to the United States.

“During a briefing for reporters Monday in the ornate Eisenhower Executive Office Building on the Administration’s efforts to address the issue of so-called foreign terrorist fighters, the official said that the latest assessment from National Counterterrorism Center is that more than 100 Americans have attempted to fight in Iraq and Syria alongside ISIS or the al-Qaeda affiliated al-Nusra Front,” Time Magazine’s Zeke Miller reported.

“It includes those who’ve gone, those who’ve tried to go, some who’ve come back and are under active—the FBI is looking at them,” the official said. “These are FBI matters, I refer you to them on specifics.”

It marked the first official government confirmation that at least some of the Americans fighting alongside the Islamist extremist group have returned to the U.S. As late as Monday morning, Secretary of State John Kerry said on MSNBC that “we have over 100 fighters there from America,” leaving out any mention of Americans who have returned.

That is not especially comforting.

First, the revelation that the FBI is conducting an ongoing investigation into suspected American jihadist fighters who have returned to the United States would seem to compromise that investigation. Second, based on the FBI’s inability to intercept Abusalha indicates that monitoring jihadists who return to the U.S. remains a significant challenge.

Yahoo News’ Olivier Knox reported on Monday that the administration has been focused on this threat posed by returning jihadists for some time, and the president chaired a special meeting of his national security team on September 17 specifically addressing this issue.

“With respect to the Americans who may be engaged in combat in Iraq and Syria, this is something that our national security agencies and counterterrorism team are taking very seriously,” National Security Advisor Susan Rice said Friday.

“It’s something we track closely. And we are doing obviously all that we can to both gather the necessary information and take the appropriate precautions to the greatest extent that we possibly can,” Rice said.

Asked whether 100 was a relatively low number that might signal success in keeping Americans off Syrian battlefields, the anonymous official replied: “I think that success is 0.”

American officials believe that there are up to 100 Americans who have traveled to Iraq and Syria to fight with Islamist militias. By some estimates, up to 2,000 Europeans are fighting in the Middle East.

On Monday, ISIS leaders called for attacks on the United States and France for executing airstrikes on Islamic State targets. “Do not ask for anyone’s advice and do not seek anyone’s verdict. Kill the disbeliever whether he is civilian or military, for they have the same ruling,” an ISIS spokesman reportedly said. “Both of them are disbelievers. Both of them are considered to be waging war.”

“Rig the roads with explosives for them,” he added. “Attack their bases. Raid their homes. Cut off their heads. Do not let them feel secure. Hunt them wherever they may be. Turn their worldly life into fear and fire. Remove their families from their homes and thereafter blow up their homes.”