A day after video surfaced of a black-hooded executioner beheading a second American journalist, National Counterterrorism Center Director Matthew Olsen played down fears that the Islamic State posed an immediate threat to the U.S. homeland.
But he did note that Europe is more susceptible to an attack because of the thousands of fighters who have traveled to join the conflict in Iraq and Syria.
"At this point, we have no credible information that ISIL is planning to attack the U.S.," he said, using one of several acronyms for the extremist group. However, if "left unchecked, they would turn their attention to the West -- and potentially to the United States."
More than 12,000 fighters have flocked to Syria over the last three years, Olsen said. Thousands hold European or other Western passports and can travel easily across many international borders. Several dozen, perhaps as many as 100, hold U.S. passports, and officials say that number is growing.
"These foreign fighters are likely to gain experience and training and return to their home countries, battle-hardened and further radicalized," he said.
This notion is an ever-growing concern for European policymakers. The returning fighters from the various Islamic militant groups embattled in the Syrian civil war, including the Islamic State and Al Qaeda, are a looming long-term menace, officials say.
On Wednesday, authorities in Bosnia detained 16 people suspected of having aligned themselves with militant groups in Iraq and Syria.
In Britain, the government warned that a terrorist attack is "highly likely" there because of the ongoing battles. Just this week, Britain proposed legislation allowing police to seize passports of travelers suspected of going abroad to fight.
A similar law is being fast-tracked through Parliament in France, where a 16-year-old girl suspected of trying to travel to Syria to join Islamist rebels was arrested along with her alleged recruiter as she passed through airport security over the weekend.
Still, here in the U.S., a "lone wolf" sympathizer could be inspired to carry out a terrorist attack through the Islamic State's barrage of graphic propaganda posted to YouTube, Facebook and Twitter, Olsen said.
"ISIL is not Al Qaeda pre 9/11," he said. "We also are not -- as a country and as a counter-terrorism community here and across Europe -- what we were pre-9/11. We are so much better postured, in so many ways, to see, detect, stop any attack like what we saw on 9/11."