The Trump administration is installing sections of border wall at a feverish pace to meet President Donald Trump’s goal of 450 miles of new barrier by the end of 2020.

The Wall Street Journal reported that the construction is being done mostly on federal land, and it involves a mix of replacing existing sections of wall with taller barriers and installing brand new sections.

Trump has long promised to build a border wall across nearly the entire border between the United States and Mexico, arguing that it will help curb illegal immigration.

Building the new wall is expensive, however. According to the Journal, one mountainous area in Arizona required the use of dynamite to build the wall at a total cost of roughly $41 million per mile.

Border Patrol Chief Rodney Scott said last week that the border wall project involves building roads for law enforcement to patrol and better lighting, and that his agents can now focus their attention on other areas.

“Every mile of border [wall] system is an investment with an immediate return on investment to the American taxpayers,” said Scott, according to the Journal.

Scott also posted an update to the border project on Twitter, saying that 360 miles of border wall have been built, 221 miles are under construction, and 157 miles are in what he called “pre-construction.”

Former Immigration and Naturalization Service Commissioner Doris Meissner, however, told the Journal that building the wall is a political move by Trump.

“At this point, the building is largely opportunistic in order to meet political demands,” said Meissner, a veteran of the Clinton administration. “A wall is not an all-purpose, one-size-fits-all solution for the challenges of border enforcement.”