SAN DIEGO — The United States does not have a way to measure how fencing works to deter illegal crossings from Mexico, according to a report released Thursday by the Government Accountability Office.

The GAO said the government spent $2.3 billion from 2007 to 2015 to extend fences to 654 miles of the nearly 2,000-mile border. 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 measure success were aborted in 2013 during a budget showdown between President Barack Obama and Congress, according to the report.

President Donald 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 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.