A U.S. official who has reviewed the files gathered on the attackers in the past week said there's growing concern that perhaps as many as three could have slipped through the U.S. watchlist and screening system.
There is no indication any of the attackers had tried to travel to the United States.
"We have seen no connection at all between the Paris attackers and the United States," FBI Director James Comey said Thursday.
Part of the issue is due to the incomplete information that European countries have on their citizens who are suspected of either radicalizing or joining terrorist groups.
As a sign that the Obama administration agrees that there are gaps that need closing, one of the U.S. officials said, in the coming days the administration expects to announce plans for additional steps to be taken with European countries that participate in the visa waiver program.
Four of the Paris attackers were on the broad watchlist of known or suspected terror suspects called TIDE (Terrorist Identity Datamart Environment), which has 1.1 million names, the U.S. officials said. At least one of the suicide bombers was on the no-fly list before the attacks.
"The vast majority of the people who were either attackers or plotters and who we are currently tracking were either watch-listed, or no-fly list, and the majority were on the no-fly list," a U.S. official said. The official included plotters, like suspected ringleader Abdelhamid Abbaoud, who were already well known to intelligence agencies, but some of the actual attackers do not appear to have attracted much attention of security officials in Europe or the U.S.
Some U.S. law enforcement officials are concerned that as more becomes known about the Paris attackers, the various U.S. watchlists and other measures may not have been enough to stop their travel.
Citizens of 38 countries, mostly in Europe, participate in the visa waiver program that allows travel to the United States for up to 90 days without visas. The travelers are still screened against U.S. security databases. In the past year, the Department of Homeland Security, reacting in part to the increase in the number of European foreign-fighters traveling to Iraq and Syria, has added new data requirements as a way to prevent extremists, including ISIS recruits, from coming to the U.S.
Non-European citizens would still need a visa and likely would not be admissible to the U.S. under current immigration laws. For instance, a refugee from Syria who is already a resident in Europe would likely be inadmissible to the U.S. and not allowed to board a plane bound for the country, a U.S. official said.
"This is always an individualized, fact-driven question, but any assumption that any plotters would have been easily able to get into the U.S. is misguided," the official said.
But there's a divide among U.S. intelligence and counterterrorism agencies about the efficacy of the watchlist system in the case of these attackers. While not all the attackers were on the watchlists, a U.S. intelligence official insists that human sources and other methods used by intelligence agencies would have filled the gaps in the watchlist system. The official declined to explain how that information would have been known.
Some U.S. counterterrorism officials dispute that likelihood and have expressed serious concerns about how little was known about the attackers.
Earlier this week, officials told CNN none of the suspected attackers was on lists maintained by U.S. law enforcement to check against traveler manifests, though they were "known" to U.S. intelligence. Officials say based on what was known initially about the attackers, there was no indication any of the individuals had made it onto the FBI-run lists.
A National Security Council spokesperson said the visa waiver program uses "multiple layers of security" to screen for threats.
"These layers of security include comprehensive screening of (visa waiver program) travelers prior to departure for the United States, at various points throughout the traveler's journey, and in-person upon arrival at U.S. ports of entry," the NSC spokesperson said. "Over the last year, the DHS and State, in coordination with a number of other federal agencies, have made a number of enhancements to the VWP to ensure our security apparatus continues to adapt in the face of evolving threats and challenges."