Rafael Carranza

azcentral.com | lavozarizona.com

Building a wall or other barrier along the entire U.S.-Mexico border would cost about $21.6 billion, and take up to three and a half years to complete, according to an internal Department of Homeland Secretary document.

The estimate is almost double the cost cited by President Donald Trump — who made a border wall built at Mexico's expense his signature issue throughout the presidential race — as well as new DHS Secretary John Kelly, who commissioned the report.

The document containing the estimate, first reported by Reuters, lays out a three-phase plan about where construction could begin along the 1,250 miles of border without physical barriers and it details challenges to constructing a wall.

But the report also leaves questions unanswered, namely what the wall will look like.

Kelly toured the Arizona-Mexico border near Nogales on Thursday and met with Gov. Doug Ducey, officials from the federal agencies under DHS and the state's four border sheriffs. One purpose of those meetings was to gauge their opinion of the usefulness of a wall.

"We talked about the value of technology in some areas and physical structures in others, but there's a lot to be vetted out in that," Pima County Sheriff Mark Napier said after the meeting.

Trump has said a border wall would cost anywhere between $10 billion and $12 billion. Senate Majority Leader Mitch McConnell told congressional Republicans during a private retreat in Philadelphia that the cost would be closer to $12 billion to $15 billion.

MORE: Family shattered by deportation vows to stay united

The higher estimate in the DHS report is due in large part to the ballooning costs of acquiring private land, including through eminent domain. Large sections of the border, particularly in Texas, are privately owned.

"People's individual property will be condemned by the federal government for construction of the wall," Denise Gilman told The Arizona Republic. "That’s what happened in the past. And that’s what would have to happen."

She led an effort in 2013 at the University of Texas Law School to document fencing already in place at the Texas-Mexico border. They found barriers erected less than a decade ago had effectively stripped owners of their property, or cut it in two.

Three planned phases

The first phase of construction would begin in September, according to the internal document, covering some 26 miles overall in San Diego, El Paso, and the Rio Grade Valley in south Texas.

The Arizona border is included in the second phase, covering 151 miles. That phase would also cover additional parts of Texas, in Laredo and the Big Bend area.

The final phase would cover the remaining sections.

RELATED: Ariz. border sheriffs press DHS chief for more resources

In all, construction would take up to three and a half years, the document states. That time frame differs from what Kelly told Fox News last week, saying the wall would be completed within two years.

Despite repeated assurances that Mexico would pay for the wall, the report assumes Congress will allocate funds later this year for the project.

Kelly is expected to present the document publicly in the coming days, according to Reuters.