With a bidding process currently under way, US border agency says between four and eight companies will make the final round and construct prototypes

Rival prototypes for Donald Trump’s wall on the US-Mexico border should be built by September, the Customs and Border Protection (CBP) agency said on Tuesday.

A bidding process for contractors to design and construct prototypes at the south-west border in San Diego, California, the first step towards the multibillion-dollar project, is currently under way.

“Our planning gets us to construction for the prototype process at the end of summer,” Ron Vitiello, acting deputy commissioner of CBP, told reporters on Tuesday. “There’s a contracting process that has to go on so we’re evaluating proposals now. We have to make decisions against that and then we’ll have to see what the planning allows for. We think it’s summer but I can’t be more exact than that ... I think that gives us until September.”

Vitiello said between four and eight companies would make the final round in what has been dubbed a “beauty pageant”. Once given notice, they 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.

Geographically, the prototypes will be “pretty close together”, Vitiello added. “There’ll probably be more than one or two of them there at the same time but we’ll have to sequence it so they’re not in each other’s way. I don’t have that plan but I know out team is working through it.”

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Final selection will take place “after a thorough review and evaluation” of prototypes constructed, he added. But the official was reluctant to give a precise deadline for when this decision will be made and the contract awarded.

San Diego was chosen for the testing because it currently has insufficient border fencing that has been breached more than 800 times in the past year, Vitiello said. It also has “mature” infrastructure and technology.

An internal report by the Department of Homeland Security (DHS), obtained by Reuters, estimated that fully walling or fencing off the entire border would cost $21.6bn and take more than three years to build. So far DHS has only identified $20m that can be redirected to the project – enough to cover the prototypes. The department has issued a budget that asks Congress to approve nearly $1.6bn for the wall in the fiscal year 2018. During the election campaign Trump repeatedly claimed that Mexico will pay for the wall but in April he tweeted: “Eventually, but at a later date so we can get started early, Mexico will be paying, in some form, for the badly needed border wall.”

In the long term, border areas will be prioritised according to facts such as census data, arrest rates and engineering and design feasibility. Vitiello acknowledged that the wall will never literally span the entire border. There are some areas where “fencing is just not practical”, he said. “It’s not necessary. The natural barrier already slows people down as they’re trying to cross the border ... So we’ve ruled out those obvious, common sense places.

“I think it’s in the neighbourhood of 130 miles where we’ve looked at because of lakes or high mountain range or because of deep canyons and it just doesn’t seem practical to have it.”

Vitiello was also asked about a surprise suggestion floated by Trump at a rally last week that the wall could be mounted with solar panels. He said: “It hasn’t impacted the planning in of itself because our role is to identify what the requirement is for where the barrier is. We’re leaning on industry to innovate and use other materials to show us what they think is possible, doable and innovative for this purpose.”

Trump began his presidential campaign two years with the pledge of a border wall to keep out Mexican immigrants, whom he branded “criminals” and “rapists”. In his first week in the White House, he signed an executive order calling for homeland security to “begin immediate construction” of the wall.