A loophole in the system allowed five of the 9/11 hijackers to overstay their visas, which allowed them to conduct their attacks.  Fifteen years later, that loophole remains open, a product of government inaction and a lack of will to close it.

Once the hijackers overstayed their visa, there was no system in place to inform authorities of the violation and no way to track the illegals to know where they were.  This allowed the terrorists to get drivers licenses so that they could board the planes and murder 3,000 people.

Washington Times:

The Government Accountability Office says five of the 19 hijackers either overstayed their visas or lost their status by breaking the terms of their permits, making them illegal immigrants in the government’s eyes.

But nobody was looking for them — or even knew they’d overstayed — allowing them to blend in with millions of other illegal immigrants living and working in the U.S. at the time, carrying driver’s licenses that let them board the airplanes they’d use to kill nearly 3,000 people.

Earlier this year the Department of Homeland Security completed its first official estimate of overstays, calculating 1 percent of visitors on basic tourist or business visas — nearly 500,000 people — stayed beyond their deadlines in fiscal year 2015.

Tracking who and where they are, however, is difficult, and it’s not clear how much effort the government puts into looking for them. The administration deported just 2,500 visa overstays in 2015.

Homeland Security officials say they collect information, including fingerprints and photos, from visitors who arrive on a visa by airplane or boat. But U.S. ports aren’t set up to collect fingerprints when people leave, creating a loophole in the system that makes it difficult to be sure someone has left the way he came.

And land borders are another issue altogether. While officers at the U.S.-Canada border do name checks and share the information with each other, they don’t collect fingerprints. And the U.S. lacks a similar agreement with Mexico, meaning there aren’t even any name checks for the millions who cross the southwest border.

U.S. Customs and Border Protection is trying a few solutions. CBP is running a pilot program at 10 airports, testing mobile fingerprint-collection devices on select flights, and from December through May, it did name checks and fingerprint collection at the Otay Mesa land port in California.

Homeland Security Secretary Jeh Johnson has set a 2018 deadline for collecting biometrics from departing passengers at the country’s top airports.

“We will have this deployed and operational by fall of 2018. The secretary’s committed to 2018 to be doing this, and we intend to do this,” John Wagner, deputy assistant commissioner at CBP, assured Congress earlier this year.

Donald Trump has made closing this loophole a priority in his immigration plan.  The Clinton campaign won't comment.

How many visa holders are we talking about?

Earlier this year the Department of Homeland Security completed its first official estimate of overstays, calculating 1 percent of visitors on basic tourist or business visas — nearly 500,000 people — stayed beyond their deadlines in fiscal year 2015.

Tracking who and where they are, however, is difficult, and it’s not clear how much effort the government puts into looking for them. The administration deported just 2,500 visa overstays in 2015.

That's clearly unacceptable and an open invitation to terrorists to evade visa restrictions.  There's no excuse for the delay, considering that the technology to track visa overstays has been around for years.  This is simply a matter of government not prioritizing a task that should have been accomplished in the days following 9/11.

That DHS says the system won't be fully in place until 2018 is ridiculous.  There should be more urgency, since our inability to track the terrorists after they overstayed their visa was a major factor in the success of the attacks.

You'd hate to think it would take another mass casualty attack to get government off its behind to close this loophole,