Department of Homeland Security Secretary John Kelly, right, talks with chief of U.S. Border Patrol, Ron Vitiello, at the Border Patrol station in Nogales, Ariz. on Feb. 9. Credit: Mamta Popat/Arizona Daily Star via AP

The “wall” that President Donald Trump has promised to erect along the nearly 2,000-mile U.S.-Mexico border could cost as much as $21.6 billion, almost double the amount he estimated on the campaign trail, according to an internal report.

A U.S. Department of Homeland draft plan, reviewed by Reuters, calls for three phases of construction that would stretch fencing and walls across 1,250 miles and take more than three years to build. Trump, who has made border security and immigration control central pillars of his agenda, has said he could build the wall for $12 billion.

A Homeland Security Department spokeswoman did not respond to a request for comment.

The $21.6 billion price tag represents the most expensive – and unconstrained – option of building barriers along the majority of the border, according to a person who has reviewed the internal cost estimate. Customs and Border Protection has different design options, which focus on an 18-foot bollard-style fence. Less costly models exist that could be quicker and equally effective if augmented by manpower and technology.

More than 650 miles of fencing meant to obstruct pedestrians and vehicles already exists along the border, backed up in some places by nearly 50 more miles of second- and third-layer fence, according to Customs and Border Protection figures. Reveal from The Center for Investigative Reporting has mapped the types of fence and their location along the border, creating one of the most detailed pictures of the current barrier.

Homeland Security Secretary John Kelly, who toured the U.S.-Mexico border this week in Arizona and is scheduled to appear today in San Diego, is expected to receive the internal report in the coming days, according to Reuters. Kelly commissioned a working group to develop estimates ahead of asking Congress for public funds to begin construction.

Reveal reported last month that an internal estimate pegged the cost ranging from $15 billion to $20 billion, and border patrol officials said that additional fencing was not needed along much of the border. It was not immediately clear if there were different drafts of internal reports. Other estimates have pegged the cost as high as $40 billion. Critics of the wall have argued that other methods, such as better screening of employers who may hire undocumented workers, would improve ways to secure the border and curb illegal immigration.

Former border officials said that upgrading barriers, such as replacing Vietnam-era steel helicopter landing mats and metal X-shaped vehicle barriers with taller concrete-filled steel columns, would help Border Patrol agents in some areas.

Kelly discussed the barrier this week during a U.S. House of Representatives Homeland Security Committee hearing, his first appearance before Congress since his confirmation last month. He told the committee that Border Patrol agents have said they would rather have a barrier that gives them a clear view into Mexico than a wall.

“We’re not going to be able to build a wall everywhere all at once,” Kelly told lawmakers.

A day after Kelly’s testimony, Trump reaffirmed on Wednesday that it would be “a real wall.”

“The wall is getting designed right now. A lot of people say, ‘Oh, oh. Trump was only kidding with the wall.’ I wasn’t kidding,” the president said while speaking at a conference for police chiefs. “Do walls work? Just ask Israel. They work – if it’s done properly.”

Andrew Becker can be reached at Follow him on Twitter: @ABeckerReveal.

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Andrew Becker is a reporter for Reveal, covering border, national and homeland security issues, as well as weapons and gun trafficking. He has focused on waste, fraud and abuse – with stories ranging from border corruption to the expanding use of drones and unmanned aerial vehicles, from the militarization of police to the intersection of politics and policy related to immigration, from terrorism to drug trafficking. Becker's reporting has appeared in The Washington Post, the Los Angeles Times, The New York Times, Newsweek/The Daily Beast and on National Public Radio and PBS/FRONTLINE, among others. He received a master's degree in journalism from UC Berkeley. Becker is based in Reveal's Emeryville, California, office.