U.S. Customs and Border Protection officials will soon begin their evaluation of President Trump’s border wall prototypes with one major question looming over the process: What happens next?

The prototype development phase concluded Thursday, kicking off a 30-day period during which immigration officials will test the durability and scalability of eight units to determine which are most likely to prevent unauthorized immigrants from compromising the barrier and crossing illegally into the U.S.

Related: The 8 prototypes for Trump's border wall (Photos)

Without congressional funding for border wall construction, multiple CBP officials told the Washington Examiner they will face a standstill as soon as their assessment of the prototypes is complete.

“What is happening in the Beltway, and with the funding, is beyond our control,” CBP Division Chief Mario Villarreal said in an interview at the agency’s San Diego headquarters earlier this month. “Our job is nothing less than to protect our nation’s borders and the people of this country, but what is occurring within the beltway, obviously we’re watching that.”

CBP spokesman Carlos Diaz told the Washington Examiner on Wednesday there is no way for the Department of Homeland Security to fund the border wall project without receiving the $1.6 billion administration officials previously asked for and have yet to receive.

Diaz’s comments came hours before a budget blueprint omitting any mention of Trump’s border wall cleared the House on Thursday, setting the stage for GOP-backed tax reform legislation.

The administration’s reliance on congressional funding for its border wall project raises questions about the initial contracts that were awarded to each of the vendors who were chosen to construct prototypes in Otay Mesa, California. When the federal government announced which companies had been selected to build prototypes, each received a contract of up to $300 million.

A CBP official later explained that most of that money would be granted only after a prototype was chosen as the base design for the updated border wall, noting that CBP may ultimately select more than one prototype.

Most of the initial construction cost between $350,000 to $500,000, leaving more than $299.5 million available for subsequent construction. Diaz, however, said CBP cannot use this leftover money to immediately begin construction on the actual wall.

One possibility floated among proponents of the border wall would be to redirect existing money for border maintenance toward construction of a new barrier in sectors that need it most. In San Diego, for example, CBP currently spends thousands to repair cement bollards that collapse when illegal immigrants use car jacks to spread the pillars apart.

“They always to have spend money on maintenance since there’s some 700 miles of border fencing. But maybe they could engage in an imaginative use of the funds they already have,” said Mark Krikorian, executive director of the conservative Center for Immigration Studies.

In December 2015, Congress passed an appropriations bill that earmarked $273 million “for operations and maintenance” on the border through Sept. 30, 2017. But even if such funds remain available, Diaz said they could not be used toward border wall construction.

“Maintenance funding is for maintenance,” he said.

The White House renewed its demand for wall funding earlier this month, including it on a list of non-negotiables that was submitted to those working on a legislative fix to the Obama-era Deferred Action for Childhood Arrivals program.

“We would expect Congress to include all the reforms in any package that addresses the status of the DACA recipients,” one White House aide said at the time the immigration priorities were released. “Other views had their fair day in the democratic process.”