Josh Siegel / The Daily Signal

President-elect Donald Trump has said he envisions a wall, or fencing, covering about half the U.S. border with Mexico because of “natural barriers.” (Photo: Staff/Reuters/Newscom)

Trump and congressional Republicans say they could use a 2006 law signed by President George W. Bush called the Secure Fence Act that mandated a minimum of 700 miles of “physical barrier” on the southern border without specifying any particular location where fencing must be built.

The law was never fully implemented, and it did not set a deadline for the fencing to be built, meaning Trump could pick up where Bush left off.

The incoming administration just needs money from Congress to do it.

“We’re going to build a wall,” Trump reiterated in his first press conference as president-elect on Wednesday. “I don’t feel like waiting a year or a year and a half. We’re going to start building.”

According to Customs and Border Protection, an agency of the Department of Homeland Security, there is currently about 654 miles of fencing along the border. Christiana Coleman, a spokeswoman for Customs and Border Protection, told The Daily Signal 36 miles of that fencing is double-layered and 14 miles have three layers.

Coleman said the fencing consists of roughly 350 miles of single-layer pedestrian fences, most which stand about 18 feet, and 300 miles of low-level vehicle barriers that can be easily bypassed by pedestrians. The fencing is not continuous. The last segment of the fencing was built in 2014, she said.

The government is obligated by statute to reach the 700-mile floor, meaning it must still build nearly 50 additional miles of fencing at minimum.

A recent report by the Congressional Research Service stated that under the law, the government can build beyond the required 700 miles.

Trump has said that he would not seek to build a wall, or fencing, across the entirety of the nearly 2,000-mile border. He said he envisions the wall covering about half the border because of “natural barriers.”

Trump has estimated the cost of the wall to be from $8 billion to $12 billion. Other estimates have put the cost at $25 billion.

Coleman noted that challenging terrain and other considerations require alternative border security tools, such as “virtual fence” technology using towers, manned and unmanned aircraft, and surveillance sensors.

Building a wall or fence in Texas is especially difficult because U.S. citizens privately own about two-thirds of the border in the state, according to a Government Accountability Office report.

The government would have to purchase land from Texans to build on it.

During his confirmation hearing this week, John F. Kelly, a retired Marine Corps general who is Trump’s nominee to lead the Department of Homeland Security, emphasized these constraints, saying that “a physical barrier in and of itself will not do the job,” and added, “it has to be really a layered defense.”