President Donald Trump is not giving up on his demand for $5.7 billion to build a wall along the U.S.-Mexico border, saying a physical barrier is central to any strategy for addressing the security and humanitarian crisis at the southern border.

Democrats argue that funding the construction of a steel barrier along 234 miles will not solve the problems. A 2018 government report warns of increased risks that the U.S. wall-building program will cost more than projected, take longer than planned and not perform as expected.

Walls and fencing now cover about one-third of the 1,954-mile-long border. Some construction began with former President Bill Clinton in the early 1990s, George W. Bush ramped up the effort in 2006 and Barack Obama built more than 130 miles, mostly during his first year in office.

Between 2007 and 2015, U.S. Customs and Border Protection spent $2.4 billion to add 535 miles of pedestrian and vehicle barriers and other infrastructure along the border.

Trump wants to extend and fortify what’s already in place. But contracting, designing and building new wall systems complete with updated technology could take years, and past experience has shown such work can be complicated and costly.

Read more here to find out how much the government has spent on barriers along the Mexican border.

AP: Past Projects Show That Building a Border Wall Is Complex, Costly