The Department of Homeland Security found in 2017 that the steel slat border wall prototype can be cut through with a saw, NBC News reported Thursday. President Trump has been championing the steel slat design as a cost-effective alternative to a concrete wall in an attempt to demonstrate his willingness to negotiate with Democrats during the government shutdown.

A design of our Steel Slat Barrier which is totally effective while at the same time beautiful! pic.twitter.com/sGltXh0cu9 — Donald J. Trump (@realDonaldTrump) December 21, 2018

V.P. Mike Pence and group had a productive meeting with the Schumer/Pelosi representatives today. Many details of Border Security were discussed. We are now planning a Steel Barrier rather than concrete. It is both stronger & less obtrusive. Good solution, and made in the U.S.A. — Donald J. Trump (@realDonaldTrump) January 6, 2019

In October 2017, six contractors commissioned by the Trump administration built eight different border wall prototypes in San Diego near the U.S.-Mexico border. Customs and Border Protection, an agency run under the Department of Homeland Security, then conducted tests on the prototypes to check for vulnerabilities. The agency produced an internal report in February indicating that inspectors were able to breach the steel slat prototype with a saw. NBC obtained a picture of the resulting hole:

The next month, in March, President Donald Trump visited San Diego to inspect the prototypes. During his visit, he noted his preference for “see-through” barriers such as the steel slat design. He’d previously argued that this feature allowed people to avoid getting hit by “large sacks of drugs” that may be catapulted from the other side.

Trump is currently refusing to sign any spending bills that do not include a $5.7 billion allotment for a barrier along the border, which has led to a weekslong partial shutdown of the government. He has repeatedly made the false claim that Democrats are requesting a steel slat barrier.

“While the design currently being constructed was informed by what we learned in the prototypes, it does not replicate those designs,” Department of Homeland Security spokesperson Katie Waldman told NBC when asked about the steel slat vulnerability. “The steel bollard design is internally reinforced with materials that require time and multiple industrial tools to breach, thereby providing U.S. Border Patrol agents additional response time to affect a successful law enforcement resolution. In the event that one of the steel bollards becomes damaged, it is quick and cost-effective to repair.”

Building Trump’s barrier would be a complex challenge given the varying terrain along the U.S.-Mexico border and would be disruptive to private property owners and wildlife. The hypothetical barrier would likely require technological enhancements such as tunnel detectors and motion-sensing cameras. Arrests for unauthorized border crossings are also at a historic low and the bulk of illegal drug imports enter the U.S. through official ports.

Customs and Border Protection spokesperson Ralph DeSio told San Diego outlet KPBS that the prototypes “were not and cannot be designed to be indestructible.” During the 2016 presidential race, Trump promised that the wall would be “impenetrable, physical, tall, powerful, beautiful.”