The Department of Homeland Security announced Thursday the winners of a contest to build prototypes for sections of President Trump's proposed border wall.

U.S. Customs and Border Protection Acting Deputy Commissioner Ronald D. Vitiello said four companies were chosen to build the prototypes, which are planned to be built in the San Diego area.

Vitiello said the prototypes are a "significant milestone" in Trump's plan to build a wall on the border with Mexico.

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The prototypes will be built out of concrete, 18 to 30 feet high and 30 feet long. Vitiello said they would each cost between "just under $400,000 and just under $500,000."

He added that four additional prototypes made of "alternative materials" will be announced next week.

The companies selected for prototype construction were Caddell Construction Co. of Montgomery, Ala.; Fisher Sand & Gravel Co. of Tempe, Ariz.; Texas Sterling Construction Co. of Houston; and W.G. Yates & Sons Construction Company of Philadelphia, Mo.

An unnamed contractor filed a bid protest, which could delay construction of the prototypes.

According to Vitiello, the concrete portions of the wall will be built 150 feet away from the actual border. The space between an initial see-through barrier on the border and the concrete wall, the "enforcement zone," will provide a "much safer environment than what we may have planned for before," said Vitiello.

Trump had initially called for a concrete wall spanning the entire border, but later said he'd consider a see-through option after hearing from Border Patrol officials.

Vitiello also said the wall would not span the entire border, but would be constructed "where it's necessary, not along the whole border."

In July, Trump told reporters aboard Air Force One that the finished wall could span 700 to 900 miles.

"It's a 2,000 mile border, but you don't need 2,000 miles of wall because you have a lot of natural barriers. You have mountains. You have some rivers that are violent and vicious. You have some areas that are so far away that you don't really have people crossing. So you don't need that. But you'll need anywhere from 700 to 900 miles," he said.

Vitiello added that prototypes were chosen based on their aesthetics, impenetrability, resistance to tampering, scaling and anti-breach properties.

Depending on the results of prototype testing, the agency could end up using all four concrete and all four alternative materials prototypes in different sections of the wall or decide to use none.

Trump made construction of a border wall a centerpiece of his campaign, and has threatened to shut down the government unless Congress funds initial construction of the wall in 2018.

Democrats successfully blocked wall funding in May's omnibus spending bill, but $1.6 billion in wall funding was added to the homeland security budget in House appropriations bills that must still clear the Senate.

The prototypes are being built with existing fiscal 2017 funds, according to Vitiello.

- This report was updated at 5:20 p.m.