More than ever, European countries are voluntarily providing the United States with large amounts of information about their citizens, particularly as those citizens attempt to travel, the nation’s top counterterrorism official said.

Compared to the summer of 2013, U.S. intelligence professionals have seen a "pendulum swing" in the willingness of European law enforcement to share information with the U.S. on European citizens, said Nicholas J. Rasmussen, director of the National Counterterrorism Center, or NCTC, on Wednesday.

Things have turned around since summer 2013, when NSA contractor Edward Snowden first disclosed some of the nation’s most closely kept secrets on surveillance capabilities. Rasmussen said that "the politics are difficult for some of our European partners” but tracking Islamic State fighters, or ISIS, has become a priority.

Rasmussen, before the House Committee on Homeland Security, said that European partners continue to differ from U.S. counterparts on the issue of bulk metadata collection. But European reservations about data sharing in more targeted investigations had “seen a dramatic improvement,” particularly in populating the NCTC’s database, called the Terrorist Identities Datamart Environment, or TIDE. It is one of the key person-of-interest watch lists that the U.S. and other countries use to track potential or suspected terrorists.

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Thanks in part to better collaboration, he said, the Turkish “banned from entry list” now includes 10,000 individuals who are primarily European citizens. Turkey is seen as the most direct route that foreign fighters in Europe use to join ISIS in Iraq and Syria.

Some of that intelligence sharing comes from tracking people in transit through new, expanded DHS powers to screen people seeking entry into the U.S., particularly those hailing from one of the 38 countries participating in the Visa Waiver Program. People attempting to enter the country without a visa have to submit information to the Electronic System for Travel Authorization, a computer system that can automatically grant visa waivers and entry.

More than 20,000 fighters have flocked to Syria and Iraq to join ISIS, including 3,400 from Western countries and 150 Americans, according to previously-submitted written testimony from Rasmussen, first obtained by the Associated Press. The figure is the highest total of foreign fighters in the conflict yet reported by U.S. officials.

Every American suspected of traveling abroad to join ISIS is the subject of an active FBI investigation, according to Michael Steinbach, assistant director of counterterrorism for the FBI. Steinbach testified that the FBI knows how many of those 150 have returned to the U.S. but said the number was classified.

Steinbach called on Congress to stop companies like Google and Apple from offering data encryption solutions to their customers, arguing that encryption makes it impossible for law enforcement to monitor terrorist or extremist talk. In principal, data encryption allows individuals to trade messages and communicate in a way so secure that even the phone provider can't intercept it. That makes it much harder for the government obtain user phone records even with a warrant.

Apple in September announced changes to their newest operating system to better encrypt user data, as previously reported by Defense One. “On devices running iOS 8, your personal data such as photos, messages (including attachments), email, contacts, call history, iTunes content, notes and reminders is placed under the protection of your passcode … Unlike our competitors, Apple cannot bypass your passcode and therefore cannot access this data … So it’s not technically feasible for us to respond to government warrants,” they say in a statement on their Website.

Google similarly announced that it would begin to encrypt data by default on future versions of the Android phone.

Steinbach said that the ability to track communications on devices was essential to current investigations of individuals related to ISIS, especially in the U.S. "Without that lawful tool, we risk an attack,” he said.

He further offered that several subjects the FBI was targeting have begun to use encryption to avoid detection and thwart investigation, but would not reveal in the open hearing the number of subjects that had "gone dark."

It is “frankly irresponsible,” he said, for companies to offer software updates that allow no lawful means for law enforcement to intercept data.

Many security experts and even some Navy SEALs argue that encryption keeps the nation safer from cyber attacks by keeping user information more secure.