WASHINGTON – President Donald Trump has recently advocated for a "steel slat" barrier along the U.S.-Mexico border after earlier pushing for a concrete wall to stop the flow of migrants.

But steel slats may not be the "impenetrable wall" Trump promised on the campaign trail in 2016. A report from NBC News said that during a Department of Homeland Security test, steel slats were sawed through in one of the wall prototypes that the president reviewed in March 2018.

Most of the prototypes were solid concrete, but two featured steel slats. Trump had indicated he favored such a design when he reviewed the samples because border officials said they wanted to be able to see what was on the other side of a border barrier. He also said transparency could keep someone from being struck by large sacks of drugs thrown over the wall.

More:From concrete wall to steel fences: A timeline of Trump's evolving border barrier concept

According to NBC News, which obtained a photo of the sawed-through barrier, the steel slats were damaged after "military and Border Patrol experts were instructed to attempt to destroy the barriers with common tools."

On Sunday, Trump said he now prefers a "Steel Barrier rather than concrete" because it is "stronger" and "less obtrusive." Pictures he has shared of his concept resemble the steel bollard fencing already in use along much of the southern border.

A Customs and Border Protection report obtained by San Diego public broadcasting station KPBS through a Freedom of Information Act request showed that all of the prototypes could be breached in one way or another.

CBP spokesman Ralph DeSio told KPBS that the wall prototypes "were not and cannot be designed to be indestructible." He said the idea was to "impede or deny efforts to scale, breach, or dig under such a barrier, giving agents time to respond."