U.S. Customs and Border Protection (CBP) has started the process of testing prototypes for a wall along the southwest U.S. border.

Eight of the sample walls were completed in San Diego in October, according to ABC News, after six companies were chosen to construct the prototypes.

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The eight prototypes have to be 18 to 30 feet high. Four are composed of concrete, while the other four are made of various other materials.

Physical tests of the wall started on Monday; however, the official testing period began on Nov. 27, according to ABC.

Physical testing of the samples includes efforts to scale or breach the walls, using tools such as jackhammers and saws.

The samples mark CPB's first efforts to build the wall, which was one of President Trump's cornerstone campaign promises.

And he appears to have made the wall's construction a priority in office. The Department of Homeland Security expedited the construction of the sample walls in August through issuing a waiver for the process.

The Department of Homeland Security (DHS) reported Tuesday in its end-of-year immigration enforcement report that the number of people turned away or caught crossing the border illegally plummeted 23.7 percent from the previous year to 526,901 in fiscal year 2017.

DHS also reported apprehensions dropped 25.3 percent, from 415,816 to 310,531. That came on the heels of CBP reporting their lowest-ever number of apprehensions in April, at 11,127.