Analysis: Reality check for Trump's border wall
Donald Trump's vow to accelerate construction of a "contiguous, physical wall" along the US-Mexican border is running into harsh Washington realities: who is going to pay for it, and how?
Mexico will not be footing the bill, officials said. Initially, that will be down to US taxpayers, starting with money already in the Department of Homeland Security's budget which amounts to a small down payment.
Then it is up to the Republican-led Congress to come up with possibly billions of dollars more, cutting money for other domestic programmes to finance the wall.
On Wednesday, Mr Trump promised "immediate construction" would begin on the border wall, telling ABC News that planning is starting now. He again vowed that Mexico would pay the US back.
It is true there is a small amount available now in the Department of Homeland Security accounts dedicated to "border security fencing, infrastructure, and technology" - 100 million dollars (£79 million) by one congressional estimate - which would allow work to begin.
So far, thanks to spending in the late 2000s, Congress provided about 2.3 billion dollars (£1.8 billion) to construct 654 miles of fencing and road blocks. But Mr Trump has promised a wall, not just fencing.
"The facts have not changed. Building a wall is the most expensive and least effective way to secure the border," said Republican representative Will Hurd, whose West Texas swing district encompasses more than 800 miles of the border.
"There's any number of complications," said former House Appropriations Committee Chairman Harold Rogers, citing obstacles such as Native American reservations and national parks and forests.
Much of the remaining 1,300 miles is very rough terrain, with steep construction costs and a limited return for the dollar.
Hundreds of miles of the border are so rugged and inhospitable that it does not make sense to even try to build.
And in Texas, almost all of the land along the border is privately owned. When former president George W Bush tried to build border fencing starting in 2006, he faced stiff opposition from local ranchers and farmers, many of whom took the government to court on plans to use their land.
In many areas along the Rio Grande the fencing is built well inside the United States, as far as a mile north of the Rio Grande, to ensure that the structure does not interfere with the flow of the river or is built on solid ground.
The middle of the channel marks the internal border, and a 1970 treaty with Mexico stipulates that structures built there should not interfere with water flow.
A popular golf course near the border in Brownsville was cut off from the rest of the city by border fencing and was forced to close in 2015.
The existing blockade - roughly 350 miles to block pedestrians and 300 miles to block vehicles - has already been built along the southern border. That fencing was constructed in the areas that are most vulnerable to illegal crossings.
"Insofar as the problem is a physical barrier, we've basically addressed that issue," said Democrat David Price, who chaired the congressional panel that funded the border fence when Democrats controlled Congress.
"This focus, this fixation on a wall and pouring untold billions of dollars into a wall, is foolishness."
Cost estimates prepared a decade ago varied widely. A 2009 Government Accountability Office analysis put costs at 6.5 million dollars (£5.1 million) a mile for pedestrian fencing and 1.8 million dollars (£1.4 million) per mile for vehicular blockades. An actual wall constructed of concrete or brick would be more costly and difficult.
Speaker Paul Ryan said US Congress will work with Mr Trump on the upfront financing for the wall. Asked about estimates that the project could cost eight billion dollars (£6.3 billion) to 14 billion dollars (£11.7 billion), Mr Ryan said: "That's about right."
Mr Trump has repeatedly promised that Mexico will pay for his wall, though neither he nor his allies in US Congress are able to articulate how.
Already, US agencies have been told to scrub their budgets for savings that could be used for the project.
The president of Mexico, Enrique Pena Nieto, is emphatic that his country will not pick up the tab.
Mr Pena Nieto said in a televised address to the nation: "I regret and reject the decision of the US to build the wall.
"I have said time and again, Mexico will not pay for any wall."