The Trump administration on Thursday announced it has chosen four companies to build concrete prototypes of the president’s much-touted border wall.

Construction of the prototypes, to take place in San Diego, is the first step in fulfilling President Trump’s campaign promise of building a “big, beautiful” wall stretching along the 2,000-mile U.S.-Mexico border.

“Today we mark a significant milestone,” said Ronald Vitiello, acting deputy commissioner of U.S. Customs and Border Protection. “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 this year.”

Congress has set aside $20 million in the current budget to build the prototypes but has not appropriated any other money for the wall. Each of the four contracts is worth between $400,000 and $500,000, Vitiello said.

The companies chosen for the concrete prototypes were Caddell Construction of Montgomery, Ala.; Fisher Sand & Gravel/DBA Fisher Industries of Tempe, Ariz..; Texas Sterling Construction in Houston; and W.G. Yates & Sons Construction in Philadelphia, Miss.

Vitiello said the agency expects to award up to four more contracts for non-concrete prototypes next week.

Construction on the concrete prototypes is expected to begin in two weeks, and should be completed this fall within 30 days after breaking ground. The prototypes will be 30 feet long and up to 30 feet high and will be located near one another, he said. They will act as a secondary barrier in a border enforcement zone that already has a fence.

Department of Homeland Security officials will spend 30 to 60 days using small hand tools to test the prototypes' resistance to tampering and penetration, Vitiello said.

Officials will consider aesthetics as well as anti-climb features and how to complement the physical barrier with technology.

“We are not just asking for a physical structure,” Vitiello said. “We’re asking for all the tools that help secure the border.”

The administration was originally expected to announce its decision in June, but the contracting process was delayed after protests from two companies that had not made the list of finalists. 

The Government Accountability Office dismissed the protests last Friday, but unsuccessful bidders now have another opportunity to file new protests, which could further delay construction.

During his visit last week to Phoenix, Trump threatened to shut down the government if Congress does not agree to fund his wall in September.

“Believe me, if we have to close down our government, we're building that wall,” Trump said during his Arizona rally. The American people voted for immigration control. That's one of the reasons I'm here, and that is what the American people deserve, and they're going to get it.”

Seven hundred miles of fencing has already been built in the most critical areas, following the 2006 Secure Fence Act under President George W. Bush. And there’s been a significant decrease in the number of illegal border crossers since Trump took office.

The government in March asked for design submissions for two types of wall: a reinforced concrete barrier wall and one made of an alternative material with see-through capability. The government specified that the wall must be insurmountable and “aesthetically pleasing in color,” at least from the U.S. side. It must also withstand digging for at least six feet below the surface.

More than 200 companies responded with proposals. The contenders were winnowed down to a secret list of about 20 finalists.

Trump earlier this summer had floated the possibility of a solar-paneled wall between 40 and 50 feet high as a way to help pay for construction.

But with less than 2 percent of the U.S. population living within 40 miles of the Mexico border, most of the electricity generated by the wall would be useless without the construction of costly transmission lines to channel the electricity to other parts of the country.

The prototypes will allow the agency to learn about what type of structure would work best along the border. They could function as permanent barriers in San Diego or be removed or relocated elsewhere, Vitiello said. 

The companies selected to build the prototypes are not necessarily the ones that will be picked to build the wall, an agency official said. Another bidding process would ensue if funding was approved for the wall itself.

“This is not a competition to build the rest of the wall,” the official said.

The companies were notified just hours before the official announcement. Executives at Caddell and Texas Sterling declined to comment on the possibility that they are designing a wall that may never be built. Representatives at W.G. Yates & Sons did not immediately respond to Post inquiries.

Thomas Fisher, president of Fisher Sand & Gravel, said in a statement that the company is "excited and grateful" to participate in the wall project. The company is headquartered in Dickinson, ND but has operations in Arizona, New Mexico, Texas and California.

Trump’s 2018 budget calls for $2.6 billion for “high-priority tactical infrastructure and border security technology.” Of that amount, $1.6 billion is for “bricks and mortar construction” and $1 billion is for infrastructure and technology, such as roads needed to access construction sites and surveillance equipment.

The House in July approved $1.6 billion in border wall funding for 2018, but Senate Democrats will likely block it in September. Even if Trump does get the money for his wall next year, he will face high hurdles -- with construction unlikely to begin until 2019 at the earliest because of the lengthy federal procurement process, said James Norton, former deputy assistant secretary in the Department of Homeland Security under Bush when the Secure Fence Act was passed.

"Even beyond 2018, the administration will be in real trouble because it hasn't sold a longer term strategy for the physical wall to both Republicans and Democrats," said Norton, now a homeland security consultant. "That's going to be a bigger challenge in getting anything built at this point."

Since the campaign, Trump has scaled back his wall ambitions, acknowledging that a continuous barrier would not be possible — nor necessary — given natural barriers such as lakes, rivers and mountains. A seamless wall is also unrealistic because of international treaty and flood-zone requirements.

The administration had hoped to add more than 100 miles of wall over the next two years, according to a Department of Homeland Security planning document. Among the “high priority” locations would be the border sectors of the Rio Grande Valley in the southern tip of Texas as well as El Paso, Tucson and San Diego.   

Of the more than 400,000 undocumented immigrants apprehended along the southern border in 2016, nearly half were stopped in the Rio Grande Valley, according to data compiled by the U.S. Border Patrol.

Customs and Border Protection said in June that it would be installing 35 new gates in the Rio Grande Valley to cover existing gaps and begin replacing fencing in San Diego and vehicle barriers in El Paso. Trump has pointed to these repairs as a sign that his wall promise was coming to life.

Customs and Border Protection had initially planned to award contracts by June 12, with construction beginning by July 21, according to a June Homeland Security inspector general’s report.