Four companies would make the final round and construct prototypes, with the awards of prototypes expected next week, Ronald Vitiello, acting deputy commissioner of The Customs and Border Protection (CBP), said on Thursday.

"CBP asked industry input for conceptual wall designs for reinforced concrete as well as alternative materials," Ronald Vitiello, acting deputy commissioner of US Customs and Border Protection," he said.

Read more: Could President Trump actually fulfill his anti-immigration promises?

Vitiello called the selection of the four companies a "significant milestone," in Trump's border improvement plan.

A bidding process for contractors to design and construct prototypes at the south-west border in San Diego, California started earlier this summer.  

The winner will be expected to finish construction of their prototypes within 30 days, with some likely to consist of reinforced concrete and others of alternative materials.

A beautiful wall

Trump pledged in his 2016 election campaign to build an "impenetrable, physical, tall, powerful, beautiful, southern border wall" between the US and Mexico, a border of about 1,900 miles (3,100 km).

He originally said the wall would cover 1,000 miles with natural obstacles filling the gaps and pledged to start building on day one, signing an executive order setting it in motion in January.

Follow the money

Mexico, Trump promised, would pay, although Mexican President Enrique Pena Nieto has been insistent it will not. Trump has since accepted that US taxpayers  will have to cover the initial funding.

In July, the House of Representatives passed 2018 spending legislation that included $1.6 billion dollars (1.4 billion euros) to begin construction of the wall. The bill is expected to spark opposition in the Senate from Democrats. 

Trump said the total cost would be $10 billion (8.9 billion euros) to $12bn, although Senate Majority Leader Mitch McConnell estimated it would cost between $12-15bn.

A study by the Washington Post estimated the cost of the president's wall would be closer to $25 billion. The 650 miles (1050 kilometers) of fencing already put up has cost the government over $7 billion so far.

An internal report by the Department of Homeland Security (DHS), obtained by the news agency Reuters, estimated that fully walling or fencing off the entire border would cost $21.6bn and take more than three years to build.

The DHS has reportedly only identified $20m that can be redirected to the project – enough to cover the prototypes.

jbh/rc (AFP, dpa)