As a businessman, Donald Trump boasted of finishing buildings "under budget" and "ahead of schedule."

But when it comes to the president's most coveted construction project — the long-promised border wall — a government report Monday warned that the Department of Homeland Security, or DHS, risks blowing through its budget and not meeting key deadlines and that the wall may not work as planned.

The report by the Government Accountability Office, or GAO, an internal government watchdog agency, found that DHS has not provided a complete analysis required to efficiently build the proposed barrier along the Mexico border — which could result in wasted taxpayer money.

"DHS plans to spend billions of dollars developing and deploying new barriers along the southwest border," the study said. "However, by proceeding without key information on cost, acquisition baselines, and the contributions of previous barrier and technology deployments, DHS faces an increased risk that the Border Wall System Program will cost more than projected, take longer than planned, or not fully perform as expected."

"Without assessing costs when prioritizing locations for future barriers, CBP [Customs and Border Protection] does not have complete information to determine whether it is using its limited resources in the most cost-effective manner and does not have important cost information that would help it develop future budget requests," the report continued.

GAO did not estimate how much money could be wasted.

The study also found that DHS has not properly documented plans for building part of the wall in San Diego. It recommended that "DHS analyze the costs associated with future barrier segments" and noted the department agreed with GAO's push for more scrutiny.

Katie Waldman, a spokeswoman for DHS, said Monday night that risks, threats and costs were taken into account during "initial prioritization" for developing barriers.

"After years of investment in a border wall system, we experienced a roughly 90 percent reduction in apprehensions in the San Diego, El Paso and Yuma sectors," she said.

Trump rooted his presidential campaign and early presidency in the idea that the southern border needed a concrete wall to secure American borders, which he said were overrun by crime and drugs. He repeatedly said Mexico would pay for it, although Mexico has refused.

Prototypes were completed in October, and Trump toured them in March, declaring the prototypes superior to existing fencing.

The same month, lawmakers passed a spending bill that included $1.6 billion in border barrier funding. White House officials, however, have suggested the entire wall project could cost $8 billion to $12 billion, and internal DHS assessments suggest that the cost could be even higher — as much as $21 billion.

In recent weeks, Trump has threatened to shut down the federal government this fall if Congress doesn't cough up more money for the wall and change the nation's immigration laws. A proposed House homeland security spending bill would provide $5 billion in border wall funding, while a Senate version offers $1.6 billion.