FILE - In this Oct. 19, 2017 file photo, crews work on a border wall prototype near the border with Tijuana, Mexico, in San Diego. A U.S. official says recent testing of prototypes of President Donald Trump's proposed wall with Mexico found their heights should stop border crossers. U.S. tactical teams spent three weeks trying to breach and scale the models in San Diego. An official with direct knowledge of the results said they point to see-through steel barriers topped by concrete as the best design. The official spoke to The Associated Press on condition of anonymity because the information is not authorized for release. (AP Photo/Gregory Bull, File)

FILE - In this Oct. 19, 2017 file photo, crews work on a border wall prototype near the border with Tijuana, Mexico, in San Diego. A U.S. official says recent testing of prototypes of President Donald Trump's proposed wall with Mexico found their heights should stop border crossers. U.S. tactical teams spent three weeks trying to breach and scale the models in San Diego. An official with direct knowledge of the results said they point to see-through steel barriers topped by concrete as the best design. The official spoke to The Associated Press on condition of anonymity because the information is not authorized for release. (AP Photo/Gregory Bull, File)

SAN DIEGO (AP) — Recent assaults by tactical teams on prototypes of President Donald Trump’s proposed wall with Mexico found their imposing heights should stop border crossers, The Associated Press has learned, a finding that’s likely to please security hawks but raise concerns about costs and environmental damage.

Military special forces based in Florida and U.S. Customs and Border Protection special units spent three weeks trying to breach and scale the eight models in San Diego, using jackhammers, saws, torches and other tools and climbing devices, a U.S. official with direct knowledge of the rigorous testing told the AP on condition of anonymity because the information was not authorized for public release.

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Each model was to be 18 to 30 feet (5 to 9 meters) high, and contractors built at or near the maximum, which is roughly twice as high as many existing barriers. Ronald Vitiello, the agency’s acting deputy commissioner, said after visiting the prototypes in October that he was struck most by their height.

The highly trained testers scaled 16 to 20 feet (5 to 6 meters) unassisted but needed help after that, said the official, who described the assaults on the wall prototypes to the AP. Testers also expressed safety concerns about getting down from 30 feet.

Only once did a tester manage to land a hook on top of the wall without help, the official said. Tubes atop some models repelled climbing devices but wouldn’t work in more mountainous areas because the terrain is too jagged.

The findings appear to challenge what Janet Napolitano, now chancellor of the University of California, often said when she was President Barack Obama’s homeland security secretary: “You show me a 50-foot wall, and I’ll show you a 51-foot ladder.”

A Customs and Border Protection report on the tests identifies strengths and flaws of each design but does not pick an overall winner or rank them. The report recommends combining elements of each, depending on the terrain. The official likened it to a Lego design, pulling pieces from different prototypes.

The report favors steel at the ground level because agents can see what is happening on the other side through mesh, and damage can more easily be fixed than concrete, the official said. With concrete, large slabs have to be replaced for even small breaches, which is time-consuming and expensive. Topping the steel with smooth concrete surfaces helps prevent climbing.

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Brandon Judd, who heads the union representing border agents, said the recommended height and steel-concrete design make sense. He said people have been able to scale the smaller border walls, which were not put to same degree of testing before construction.

“Not many people are going to attempt to go over 30 feet,” said Judd, president of the National Border Patrol Council. “I just don’t see it happening.”

Just as daunting as getting over, he said, is climbing back if someone decides to try to return to Mexico to avoid capture.

Taller barriers are undoubtedly more effective, but they drive up the cost and could endanger wildlife.

Brian Segee, an attorney for Center for Biological Diversity, which has sued to block construction, said border walls 15 feet (5 meters) or less have prevented the movement of low-flying birds and insects.

“The bigger, more impervious the wall, the worse the impacts are going to be for wildlife,” Segee said.

U.S. Rep. Henry Cuellar, a Texas Democrat in a border district, said a 30-foot (9-meter) wall would increase the cost “tremendously” and do nothing to address the problem of people entering the country legally and overstaying their visas.

Customs and Border Protection leaders were set to be briefed on the findings this week amid a standoff over immigration legislation that threatens to shut down the government. Democrats insist it includes protections for hundreds of thousands of young immigrants who were shielded from deportation under an Obama-era program, Deferred Action for Childhood Arrivals, which is scheduled to end in March.

The administration has insisted wall funding be part of any immigration deal, but Trump has been unclear about how long the wall would be and how it should be designed. The administration has asked for $1.6 billion this year to build or replace 74 miles (118 kilometers) of barriers in Texas’ Rio Grande Valley and San Diego and plans to request another $1.6 billion next year.

A proposal by Customs and Border Protection calls for spending $18 billion over 10 years to extend barriers to cover nearly half the border. Mexico has steadfastly rejected Trump’s demand that it pay for the wall.

The agency is still in “the testing phase” and results are being evaluated, spokesman Carlos Diaz said. Combining elements of different prototypes instead of picking a winner is consistent with previous statements by officials, he said, noting that the agency said in bidding guidelines that a minimum height of 18 feet (5 meters) would be a key characteristic.

Contractors were awarded between $300,000 and $500,000 for each prototype. They were built last fall in a remote part of San Diego to guide future construction of one of Trump’s signature campaign pledges. Four were concrete and four were made of other materials.