The Department of Homeland Security said Thursday it's chosen four companies to build concrete prototypes for sections of President Donald Trump's proposed border wall, calling it "a significant milestone."

Construction of the prototypes will take place in San Diego.

"Today we mark a significant milestone," Ronald Vitiello, acting deputy commissioner of U.S. Customs and Border Protection, The Washington Post reported.

"This is the first tangible result of the action planning that has gone on. This is the use of the resources we had available for [fiscal 2017]."

Vitiello said each concrete prototype will be 18 to 30 feet high and 30 feet long, and cost between "just under $400,000 and just under $500,000," and that a set of prototypes made of "alternative materials" will be announced next week, The Hill reported.

The companies chosen are: Caddell Construction in Montgomery, Alabama, Fisher Sand & Gravel/DBA Fisher Industries in Tempe, Arizona, Texas Sterling Construction in Houston, and W.G. Yates & Sons Construction in Philadelphia, Missouri, the Post reported.