SAN DIEGO — The United States does not have a way to measure how well fencing works to deter illegal crossings from Mexico, according to a report released Thursday by Congress’ main watchdog as President Donald Trump renewed his pledge to build “a great wall” on the border.

The Government Accountability Office said the government spent $2.3 billion from 2007 to 2015 to extend fences to 654 miles of the nearly 2,000-mile border and more to repair them.

Despite those investments, the Customs and Border Protection agency “cannot measure the contribution of fencing to border security operations along the southwest border because it has not developed metrics for this assessment,” the agency said in a 75-page review.

Efforts to better measure success were aborted in 2013 because of a budget showdown between President Barack Obama and Congress, according to the report, which recommends developing new measures to justify more spending.

Trump, speaking at a news conference Thursday, reiterated plans for a wall with Mexico – one of his signature campaign pledges – and promised to negotiate a lower price.

Border Patrol leaders have struggled to say with any degree of precision how well fences work, in part because it’s unknown how many people get away. Another unknown is the extent to which fences or other factors such as the number of agents explain why people are caught.

The GAO estimated capture rates in areas with and without fencing but cautioned that no cause-and-effect relationship has been established.

Construction cost estimates have varied widely. The GAO report stuck with its 2009 estimate of an average of $6.5 million a mile for a fence to keep out people on foot and $1.8 million a mile for vehicle blockades. There are currently 354 miles of pedestrian fencing and 300 miles of vehicle barriers.

Republican leaders in Congress have said Trump’s wall would cost between $12 billion and $15 billion. Trump has suggested $12 billion.

An internal Homeland Security Department report prepared for Secretary John Kelly estimates the cost of extending the wall along the entire U.S.-Mexico border at about $21 billion, according to a U.S. government official who is involved in border issues.