The wrong type of border wall could actually make Border Patrol agents less safe, according to a new report Thursday from the government’s chief watchdog, which said walls that block lines of sight can become ambush sites for attackers looking to get the drop on agents.

President Trump has vowed to build a wall on the U.S.-Mexico border, though he has given different descriptions of what the wall would look like.

But his plans may have taken a hit from the new Government Accountability Office report, which said Customs and Border Protection doesn’t even calculate how well the existing fencing along 654 miles of the U.S.-Mexico border does in cutting down illegal immigration right now.

“CBP collects data that could be useful to assessing the contributions of border fencing to border security operations at the sector level, but has not conducted such an assessment,” the GAO investigators said in their new report.

GAO also said that solid fencing can actually be a danger to agents, with attackers on the Mexican side using it to hide and throw rocks and other missiles onto the U.S. side. Modern-style fencing that allows lines of sight, though, “has helped reduce agent assaults.”

Just as bad are breaches in the fence. More than 9,200 breaches were detected between 2010 and 2015 — with older-style fencing being cut at a rate of about 80 per mile of fencing, or six times the rate of newer designs.

Some holes are big enough to drive drug-loads through, while others allow people to sneak through. Sometimes the breaches are so bad — particularly from digging underneath — that new soil has to be dumped in, investigators said.

Primary fencing costs nearly $5 million per mile.

Adding enough fencing to go from the existing 654 miles to the entire border would cost $6.3 billion, at that rate. But the actual final figure will depend on the design Mr. Trump settles on.

In its official response, CBP said it will take more than a year to get a set of yardsticks in place to measure border security, including effectiveness of the fence.

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