WASHINGTON — (CNN) A top U.S. counterterrorism official in charge of ensuring terrorists do not make it into the United States said European countries can do more to screen terrorists because they ignore tools the U.S. has offered in the fight against terrorism.

“It’s concerning that our partners don’t use all of our data,” said Terrorist Screening Center Director Christopher Piehota in an exclusive interview with CNN. “We provide them with tools. We provide them with support, and I would find it concerning that they don’t use these tools to help screen for their own aviation security, maritime security, border screening, visas, things like that for travel.”

Europe does not make use of a central terror database and each country has its own terrorist watch list that comes with unique sets of standards for tracking terrorists. Complicating matters even more is that 26 European countries operate inside the “Schengen zone” where there are not routine border checks.

Piehota said the U.S. shares information about potential terrorists in a real-time basis with European countries so potential terrorists can be checked and officials processing migrants are able to pick out people for enhanced screening to make sure they can weed out potential threats.

However, European countries don’t consistently and systematically use the U.S. terror watch list even though they could. U.S. counterterrorism officials tell CNN those inconsistencies in data cross-referencing and privacy standards in Europe give terrorists the advantage.

Piehota says all European countries cooperate with the United States to varying degrees and information sharing has greatly improved in the wake of the ISIS threat.

He says the daunting task of making sure no terrorist makes it into the United States from abroad has become more challenging with the evolving ISIS threat in Europe where people go back and forth to Syria to train with ISIS undetected.

“There are many that we do know about. And unfortunately there are some that we do not know about.”

His biggest concern are those he doesn’t know about from visa waiver countries who could possibly slip into the U.S. to launch an attack.

Counterterrorism officials like Piehota also worry about the ISIS terrorists believed to still be in Europe and possibly plotting future attacks. One such person is the man in the hat seen in the Brussels airport surveillance video who remains on the run.

“It’s highly concerning,” he said. “We make sure that we know as much as we can. And we take that information and we use it the best we can to minimize threats to our communities. But we can’t know everything all the time.”

European officials have acknowledged the gaps in coverage and communication in the weeks following the Brussels attacks. Rob Wainwright, the director of the European Police Agency known as Europol said, “The fragmented intelligence picture around this dispersed community of suspected terrorists is very challenging for European authorities.”

The EU’s Counter-Terrorism Chief Gilles de Kerchove told CNN that he was aware of problems in getting member states to act, saying, “I do my best to put pressure, to confront them with blunt figures, and we are making progress, but not quickly enough.”

Piehota also echoed the sentiment from FBI Director James Comey that there’s risk with the U.S. plan of allowing 100,000 refugees into the U.S. a year by 2017 partially because of the lack of intelligence on people in Syria.

“Nothing we can guarantee is at a 100% level,” Piehota said, adding he believes the vetting process is rigorous with a layered approach of screening, evaluation and assessment for the refugees who will be cross-referenced with all the U.S. watch lists.

Piehota tried to set the record straight on what he called “incorrect perceptions” about the U.S. watch lists, many that have been fueled by presidential candidates on the campaign trail who have claimed a majority of people on the lists are innocent Americans.

“U.S. persons on the watch list comprise less than half a percent of the total populations. Very small, controlled population” he said, adding that those on the list are consistently re-evaulated with 1,500 changes to the list on average per day, including people being added and removed from the list, and records being updated. Piehota wouldn’t say how many people are currently on the U.S. terror watch lists other than “a lot.”

In order to make it on to one of the terror watch lists that were formed after the September 11, 2001, terror attacks, authorities must have “reasonable suspicion” that person has ties to terrorism. That standard has been cause for criticism from civil liberties groups who believe the bar should be higher to be put on the list.

In response, Piehota said, “If we make the standards too high, we’re going to miss people, potentially, who are suspected terrorists who may be in the planning or plotting phases, we will miss them. If we make it too low then we will impact the privacy and the civil liberties of the public, so we use the reasonable suspicion standards as a compromise between those two extremes to make sure that people are screened against a standard, that we consider to be suitable for watch listing.”

To make it onto the no-fly list the standard there’s more scrutiny — “you have to be able to show that you present a certain kind of threat to the United States, whether it’s domestic, whether it’s aviation, overseas, or you present a threat of being operationally capable of conducting an act of terrorism.”

When asked about whether anyone could be watchlisted based on being a Muslim as presidential candidate Donald Trump has proposed, Piehota would only say “no watch listing activity is conducted based up on their race, religion, or any other protected right. There has to be a certain level of derogatory information, particularized to that individual that would warrant their watchlisting.”

The European Union has been examining the sharing of passenger information for at least six years, and the European Parliament is expected to consider a measure related to that issue this month. Some members oppose the idea on privacy grounds.

“That will require difficult discussions with European Parliament, because we’re sensitive about balance between security and freedom,” de Kerchove said last month.

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