More than a third of Visa Waiver Program countries have failed to share information about known or suspected terrorists with the United States, according to a new report released by the Government Accountability Office (GAO).

The Visa Waiver Program allows foreign nationals from 38, largely European, countries to travel to the U.S.  without a visa for tourist and businesses stays up to 90 days. The program requires participating countries to enter into information sharing agreements to report travel threats. The GAO reports, however, that many VPW countries are not living up to their end of the bargain.

According to GAO, while all 38 countries have reported missing passports, more than a third have failed to share terrorist identify information through a second program agreement and more than a third have not shared criminal history information through a third program agreement.

“While VWP countries may share information through other means, U.S. agency officials told GAO that information sharing through the agreements is essential for national security,” the report reads.

The VWP has come under scrutiny in recent months and years due amid Islamic terrorist attacks and the growing presence of jihadi terrorist sympathizers and conspirators in Europe.

“The Department of State (State) has reported that in recent years, thousands of foreign terrorist fighters—including many from VWP countries—have traveled to countries such as Syria and Iraq to train with, support, or join extremist groups, such as the self-proclaimed Islamic State in Iraq and Syria (ISIS), that are hostile to the United States,” GAO notes.

In addition to the information sharing concerns, GAO also identified gaps in the Department of Homeland Security’s VWP evaluations, highlighting that the agency has been tardy or failed to submit a number of reports to Congress.

“Nonetheless, as of October 31, 2015, GAO found that about a quarter of DHS’s most recent VWP congressional reports were submitted, or remained outstanding, 5 or more months past the statutory deadlines,” the GAO report reads. “As a result, Congress may lack timely information needed to conduct oversight of the VWP and assess whether further modifications are necessary to prevent terrorists from exploiting the program.”

House Homeland Security Chairman Michael McCaul (R-TX) called the findings in the report “disturbing” and called for more pressure to be applied to VWP countries to comply with the agreements.

“Thousands of Europeans have gone to fight in Syria and Iraq, and most of them are from countries that have visa-free access to the United States,” McCaul said in a statement Tuesday. “These extremists are only a plane-flight away from our shores, which is why overseas counterterrorism cooperation is critical. But if our partners do not share data about terror suspects quickly and transparently, such fanatics might be able to slip through the cracks to enter our country.”

In December the House passed legislation aimed at tightening up the VWP. The legislation — Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015 — was signed into law in conjunction with the year-end spending bill. Among its provisions was the requirement that all VWP countries fully implement the information sharing agreements.

McCaul urged DHS to “aggressively” implement the legislation.