The four bid winners, announced by the US Customs and Border Protection (CBP) on Thursday, are Caddell Construction from Montgomery, Alabama; Fisher Sand & Gravel in Tempe, Arizona; Texas Sterling Construction from Houston and W.G. Yates & Sons Construction Company in Philadelphia, Mississippi.

The companies have now been requested to build prototypes of their designs for the wall at a location near San Diego, the CBP said, adding that construction may begin in September and last for 30 days.

Read more: German firms back off from building Trump's border wall

Ronald Vitiello, acting deputy commissioner of CBP, told a news conference in Washington that each prototype must be 30 feet (9.1 meters) long and between about 18 feet and 30 feet high. The government would pay as much as $500,000 (420,000 euros) each for the mock-ups.

CBP said in a statement that prototyping was an "industry-tested approach" aimed at identifying the best solution when considering a new product or methodology. "Through the construction of prototypes, CBP will partner with industry to identify additional means and methods to construct a border wall," the statement added.

CBP officials said that after an evaluation there could be multiple winners. The agency is scheduled to announce winners for a second bid, for other types of wall designs, as soon as next week. Those prototypes would also be reviewed before any final decision was made, Vitiello said.

Wall aesthetics matter

In March, CBP issued two requests for proposals to acquire conceptual wall designs. A bid protest that had delayed the award was dismissed on August 25. Further protests from builders that weren't selected are possible.

Vitiello told journalists that CBP would look at the "aesthetics" of the design proposals, but also at "how penetrable they are, how resistant they are to tampering and then scaling or anti-climb features." The next step would then be to "sit down with these companies and discuss timing," Vitiello added.

US President Donald Trump first promised to build a wall at the Mexican border to check illegal immigration when he launched his presidential campaign in June 2015 and made it a central pillar of his campaign. He also pledged that Mexico would pay to build the structure.

On June 27, the US House of Representative passed a spending bill that complies with Trump's demands to start work on the wall. Republican leaders added $1.6 billion in border-wall funding for the fiscal year that begins October 1. The Senate hasn't taken up the measure, prompting Trump last month to threaten a government shutdown if lawmakers refuse to send him a spending bill that funds the wall.

Trump has repeatedly said he wants to build a "big beautiful wall" with a "big beautiful door." But he hasn't said how long the barrier would be. Currently, there is roughly 650 miles of fencing already in place along the border, nearly all of that fencing can be seen through. Officials say a solid concrete wall would only be built alongside existing fencing.

uhe/tr (Reuters, Bloomberg)