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The US Department of Homeland Security (DHS) has issued requests for proposals for prototypes for a wall along the Mexican border.

According to fortune.com, a wall to stem illegal immigration was one of Donald Trump’s main campaign promises and has been highly controversial. The president has vowed to make Mexico reimburse the United States for its cost but Mexico has repeatedly said it will not do so.

The requests for proposals are another step toward fulfilling Trump’s campaign promise of building a wall on the US southern border to keep out Mexican immigrants. The DHS requests specify that ideally, the wall should be 30 feet (9 meters) high and the wall facing the US side should be “aesthetically pleasing in color.”

According to one document posted online by U.S. Customs and Border Protection, the wall should be built using concrete, and “physically imposing.” However, it says designs over 18 feet (5.5 meters) high could be acceptable.

“Designs with heights of less than 18 feet are not acceptable,” the document said. It said the wall should have features that do not allow people to climb over it and should prevent digging below the wall.

“The wall shall prevent/deter for a minimum of 1 hour the creation of a physical breach of the wall (e.g., punching through the wall) larger than 12 inches (30 cm) in diameter or square using sledgehammer, car jack, pickaxe, chisel, battery-operated impact tools, battery-operated cutting tools, oxy/acetylene torch or other similar hand-held tools,” it said.

Although the wall program has been consistently opposed by Mexico, Mexican Americans, and many of the Americans, theguardian.com reported that about 10% of the construction firms interested in building the first stage of the wall are Hispanic-owned. In a review of the more than 600 companies that submitted proposals to the Department of Homeland Security, 62 were Hispanic-American businesses.

A few days earlier, the White House requested $3 billion more for Homeland Security, with some of that intended for planning and building the border wall.