The Department of Homeland Security (DHS) has taken a major step in fulfilling a directive laid out in a January executive order by President Donald Trump to build the U.S.-Mexico border wall. Yesterday, DHS awarded contracts to four firms to build concrete prototypes.

According to The New York Times, “The companies selected will each have 30 days to complete the projects after the agency gives them the order to proceed.” Each prototype will be a concrete wall approximately 30 feet high. Writes The Times:

Homeland Security officials said the prototypes would be added to the existing walls in the San Diego area and would allow the agency to evaluate which barriers are most effective in giving Border Patrol agents time to respond to reports of illegal drugs and human smuggling.

The nearly 2,000-mile border wall was a key element in Donald Trump’s campaign. He promised they would keep out “bad hombres,” as he called them, from Mexico. In July, it was reported that soil testing and related work had been underway for six months in the area where the proposed first segment is to be built, which runs through the Santa Ana Wildlife Refuge. As Colorlines previously wrote, “Construction can begin at the 2,088-acre refuge—home to 400 bird species and the endangered ocelot—as early as January 2018.” Last month, the Trump administration said, according to two government sources, that it will not conduct an environmental review of the impact the wall will have on Santa Ana.

Trump requested that Congress allocate $1.6 billion this year toward the wall. As Congress decides the budget, The Times reports that in the immediate aftermath of Hurricane Harvey, “Some congressional critics have accused the administration and its Republican allies of proposing to cut billions in funding to the Federal Emergency Management Agency, the lead department for disaster recovery, to pay for a border wall.”

DHS requested proposals for two categories of walls: concrete and other types. Next week, the agency will announce who received contracts for proposals using alternate materials.