A report from a government watchdog agency has concluded that "key information" is missing from plans to build a border wall along the US-Mexico border.
President Donald Trump has promised a border wall since he began his campaign, and in July threatened a government shutdown if Congress doesn't provide funding to build it.
The report warned the project could cost far more than estimated or that the wall would "not fully perform as expected."
President Donald Trump's long-promised border wall between the US and Mexico could costs billions of dollars more than estimated, a new report from a government watchdog agency said on Monday.
The report from the Government Accountability Office, a nonpartisan group that forms part of Congress' oversight arm, said that "key information" is missing from the plans.
It further concluded that the US Department of Homeland Security "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."
Factors like a varying terrain and land ownership had not been taken fully into account, the report said, which drive up costs.
The overall cost of the wall is unknown, so the report simply notes that the administration plans to spend "billions" on its construction and calls for Customs and Border Control to analyze the costs for future sections of the wall.
Locations for future sections of the wall had been identified without factoring in where they would be needed to prevent illegal border crossings, the report also concluded.
It said US Customs and Border Control "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."
A Department of Homeland Security spokesperson told CNN that it would be "misleading and inaccurate" for the Government Accountability Office to say that "progress is not being documented or to imply that progress is not being tracked."
In January, Trump wanted $25 billion for his wall, but Congress shot him down and provided just $1.6 billion. In July, he threatened a government shutdown if Congress does not provide funding to build the wall.
Conflicting estimates of how much it would cost to build barriers along the entire 1,933-mile border have marked Trump's quest to build the wall since he first proposed it during his presidential election campaign.
An internal report that Reuters obtained in February 2017 said the wall would cost as much as $21.6 billion. Other estimates have placed the costs higher, with Democrats asserting it could reach $70 billion to build.
Rep. Bennie Thompson, a Democratic Member of the House Homeland Security Committee, said in a statement that the report showed the Trump administration was "moving too fast" with the wall and has "no clue what it is doing."