Complaints against U.S. Border Patrol agents obtained by the ACLU of Arizona and The New York Times suggest a pattern of arbitrary detention, racial profiling, fruitless vehicle searches, and excessive reactions to perceived disrespect:

Last year, in southeastern Arizona, a military veteran said his children shuddered with fear in the back seat as agents repeatedly asked him if the children were really his. A woman at a checkpoint between Phoenix and Tucson said an agent threatened to use a stun gun on her brother in 2012 after he asked why their vehicle was being searched. And at a California checkpoint in 2013, a man said an agent approached him, hand on his holstered weapon, and demanded, "How would you like to have a gun pointed at your face?"

If you think that Taser threat was a joke, think again. A jolt from a stun gun is beginning to look like a standard punishment for uppity motorists.

These and many other incidents described in the complaints occurred not at the border but at internal checkpoints that are ostensibly aimed at enforcing immigration laws but more often seem to facilitate drug searches. The Supreme Court has said no justification is needed to stop a vehicle at one of these checkpoints, which can be located anywhere within 100 miles of an "external boundary"—a zone that includes about two-thirds of the U.S. population. But after a "response to a brief question or two and possibly the production of a document evidencing a right to be in the United States," a driver should be free to go, unless an agent has a reasonable basis to believe the vehicle is carrying illegal immigrants or contraband. Even if an agent reasonably suspects illegal activity, he cannot legally search a car without probable cause to believe he will find evidence of it.

Luckily for curious Border Patrol agents (and cops of all kinds), the Supreme Court says a dog's as good as a warrant. "Agents rely heavily on drug-sniffing dogs to inspect the thousands of cars that go through the Border Patrol's busiest checkpoints," the Times notes. "But the agency does not seem to keep track of when dogs alert, how often they alert and how often their alerts are wrong. It records an alert only when it results in an arrest."

Such selective recordkeeping also has been blessed by the Court. Although a defendant theoretically can challenge a search by arguing that the dog whose alert allegedly justified it was unreliable, law enforcement agencies have no obligation to collect the data required for such a challenge and every incentive not to do so. It is in any case not at all clear what kind of evidence would be required to show that a dog's purported alert should not count as probable cause, especially since the government routinely argues that anything resembling a false alarm is a reaction to otherwise undetectable traces of drugs.

It does not help that the people bringing such challenges are almost always guilty. The average innocent motorist who is inconvenienced and humiliated by a fruitless search is unlikely to file a lawsuit over it, or even a complaint.

Given that reality, the 142 complaints that the ACLU obtained by suing the Department of Homeland Security—which includes Customs and Border Protection (CBP), part of which is the Border Patrol—and the "hundreds of other cases" identified by the Times probably should be viewed as the tip of a very dirty iceberg. The Times notes that CBP reports to Congress mentioned "three complaints accusing agents of violating the Fourth Amendment" in fiscal years 2012 and 2013. Yet the ACLU found "81 such accusations filed during the same period against agents assigned to the Border Patrol's Tucson and Yuma sectors," which are just "two of its 20 regional divisions along the southern and northern borders."

The true extent of the Border Patrol's Fourth Amendment violations may be unknowable, because an agent can always claim a dog alerted, whether it did or not, and in practice that's all it takes to search a car. "The tired excuse of 'the dog alerted' has worn incredibly thin as a reason to search Hispanics," noted the author of one Border Patrol complaint, who works as a lawyer for the city of Nogales. That's true for the Border Patrol's victims, but so far it is not true for the courts.

Addendum: Isaac Skelton notes on Facebook that the Supreme Court last April said police may not extend a traffic stop merely for the purpose of facilitating a canine inspection. In that case, Rodriguez v. U.S., the Court held that in the absence of reasonable suspicion, an eight-minute delay is enough to make a driver's detention unconstitutional. As I argue here, here, and here, Rodriguez has important practical implications. But in the context of a Border Patrol checkpoint already equipped with drug-sniffing dogs, I suspect the rule established by Rodriguez would rarely make a difference.