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 today, 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. The official spoke on condition of anonymity because the report has not been made public.

The Homeland Security report proposes an initial phase that would extend fences 26 miles and a second wave that would add 151 miles, plus 272 “replacement” miles where fences are already installed, according to the official. Those two phases would cost $5 billion.

Few people dispute that fences contributed to a sharp drop in crossings in cities like San Diego and El Paso, Texas, where people can easily blend in once they enter the country. Before fences were built in San Diego, crossers played soccer on U.S. soil as vendors hawked tamales, waiting until night fell to overwhelm agents.

San Diego was the busiest corridor for illegal crossings until the late 1990s, when an enforcement surge pushed traffic to Arizona and other more remote areas where many crossers died from heat. As fencing critics note, border crossers continue to perish in isolated areas under extreme weather conditions.

Border Patrol agents the told authors of the GAO report that fencing has made it more difficult for people to ambush or assault them. Attacks on agents dropped 81 percent two years after fencing was erected in the Nogales, Arizona, area.

On the flip side, holes are often cut. The GAO reported 9,287 breaches in pedestrian fencing from 2010 to 2015.

Kelly told lawmakers last week that he would like to see wall construction well underway within two years, but he held open the possibility that it would not extend to areas where natural physical barriers already exist.

After a tour of the border in Arizona and California, he said he got “an earful” of suggestions from employees on where to build first.

“I’ll take that on board. We’ll bring it back to Washington, put in the blender and come up with a solution,” he said.