Fifteen years after the September 11 terrorist attacks, the United States government still has little idea how many foreign travelers overstay their visas annually and remain in the U.S. This, despite the fact that two September 11 hijackers, Satam al-Suqami and Nawaf al-Hazmi, overstayed their visas before carrying out the deadly attacks that left nearly 3,000 Americans dead. Two other 9/11 hijackers had also previously overstayed their visas.

Foreign travelers overstaying their visas to remain in the U.S. illegally represents one of the gravest national security threats to the homeland. We know the risk posed by not enforcing our visa laws—it’s unacceptable that we are turning a blind eye to these violations.

Federal law requires that agencies of jurisdiction annually report to Congress an estimate of the number of individuals who have overstayed their visas. However, in flagrant violation of that Congressional mandate, neither the Department of Homeland Security (DHS) nor its predecessor has submitted a report since 1994. 

After numerous Congressional requests and inquiries, DHS released a report this past week indicating that 500,000 foreign travelers overstayed their visas in 2015 to remain in the U.S. illegally. 

However, that figure does not represent the full number of visa overstays in the U.S., since the data is only for those who came into the United States through certain modes of entry and does not include some visa types including student and fiancé visas. Tashfeen Malik, who carried out the deadly terrorist attack with her husband in San Bernardino, had come to the U.S. on a K-1 or fiancé visa.

And it gets worse.

Since 1996, Congress has required the agency of jurisdiction—now DHS— to set up a biometric exit system to track visa overstays.

Yet here we are in 2016 and it has yet to be implemented. Exit checks are absolutely critical to monitor if those who come into the U.S. on visas actually do in fact leave the country when their visa is up. Estimates reveal that at least 40 percent of illegal immigrants come to the U.S. on temporary visas and simply overstay.

Annually, Congress has appropriated hundreds of millions of dollars to go towards the implementation of the biometric exit program, yet it still isn’t operational.

It’s absolutely ludicrous that these basic security measures have not been put in place to track the flow of visitors coming and going in and out of our country. It’s not just irresponsible of DHS—it’s inexcusable.

When we talk about the national security threat posed by visa overstays, we are not speaking in hypotheticals. We are facing the reality of recent history.

Two of the 9/11 hijackers exploited the security vulnerabilities in the United States’ visa program and were able to carry out their attacks. It is astounding that over a decade later DHS still has done little to fix the problem.

Republican Mark Meadows represents North Carolina in the U.S. House of Representatives and serves on the Foreign Affairs Committee.