Until recently, Kilo, a drug-detecting dog employed by the Moffat County, Colorado, sheriff's office, had a good, reliable gig, justifying searches by alerting to vehicles cops deemed suspicious. But a state appeals court ruling published last week casts doubt on Kilo's future. The decision confirms that Kilo, like every other conventionally trained drug dog in the eight states that have legalized marijuana, is not nearly as useful as he used to be. He might need retraining. He might even be out of a job.

As the Colorado Court of Appeals explained, marijuana legalization raises two problems for dogs like Kilo. It undermines the legal logic that says letting him sniff a car does not count as a search, and it casts further doubt on the already dubious notion that his bark (one of the signals he uses to indicate the presence of contraband) by itself supplies probable cause for a search.

The case involves a 2015 search of a truck driven by Craig, Colorado, resident Kevin McNight. Craig police Cpl. Bryan Gonzales said he followed McNight because he saw him park in front of a house where police had found drugs seven weeks before. After pulling McNight over for making a turn without signaling, Gonzales said, he recognized his passenger as a methamphetamine user. At Gonzales' request, Sgt. Courtland Folks, Kilo's handler, brought the dog to the traffic stop. After Kilo alerted to the truck, a search turned up a "glass pipe commonly used to smoke methamphetamine." Based on that evidence, McKnight was convicted of possessing drug paraphernalia and a controlled substance (the residue found in the pipe).

When Gonzales decided to bring in Kilo without any real evidence that McKnight was carrying illegal drugs, the officer was relying on precedents that say no such evidence is necessary. According to the U.S. Supreme Court, an olfactory inspection by a drug-sniffing dog does not qualify as a search under the Fourth Amendment, which means police can perform one anytime they have legally detained someone—during a routine traffic stop, for example. But that conclusion is based on the assumption that such an examination reveals nothing more than the presence or absence of contraband and therefore does not impinge on any constitutionally protected privacy interest. That is not true if a dog might be triggered by something other than contraband, such as an ounce or less of marijuana, which in Colorado has been legal for adults 21 or older to possess since December 2012.

As is typicallly the case with police dogs, Kilo was trained to detect several controlled substances, and his alerts do not distinguish among them or indicate the amount of drugs that might be present. "It is no longer accurate to say, at least as a matter of state law, that an alert by a dog which can detect marijuana (but not specific amounts) can reveal only the presence of 'contraband,'" observes Judge John Dailey in his majority opinion. "A dog sniff could result in an alert with respect to something for which, under Colorado law, a person has a legitimate expectation of privacy, i.e., the possession of one ounce or less of marijuana for personal use. Because a dog sniff of a vehicle could infringe upon a legitimate expectation of privacy solely under state law, that dog sniff should now be considered a 'search' for purposes of article II section 7 of the state constitution [the search-and-seizure provision] where the occupants are twenty-one years or older."

Judge Michael Berger agreed with Dailey on that point, while Berger and the third member of the appeals court panel, Judge Jerry Jones, agreed on a related issue: Since Kilo's alert might indicate nothing more than the legal possession of marijuana, it could not by itself provide probable cause for a search, which requires a "fair probability" that contaband will be discovered. "A drug-detection dog's alert does not alone give a Colorado state law enforcement officer probable cause to conduct a search of a vehicle where the occupants are at least twenty-one years old," Berger writes in his concurring opinion. He concludes that even with the additional evidence cited by Gonzales (the location where McKnight briefly parked and his passenger's past meth use), the officer did not have probable cause for a search.

The problems illustrated by this case will be coming up again and again if police continue to use conventionally trained dogs to justify drug searches, not just in Colorado but also in the seven other states where marijuana is legal for recreational use (Alaska, California, Maine, Massachusetts, Nevada, Oregon, and Washington). Police will have to stop relying on such dogs or use their alerts only in combination with other evidence.

When I covered this issue in 2015, I asked Lawrence Myers, a veterinarian and neurophysiologist at Auburn University who is an expert on dogs' olfactory capabilities, whether the animals can be retrained so they no longer react to marijuana, as some police departments intended to do. "Retraining is possible," Myers said, "but it takes time and scientifically valid testing to show that the dogs no longer alert to marijuana." He added, "I doubt that many departments would do the testing."

That concern is fully justified given the uneven training and lax testing that make the "well-trained narcotics detection dog" of the Supreme Court's imagination more an aspiration than a reality. Even with proper training, a dog may respond to its handler's expectations, distracting stimuli, or smell-alike cues rather than actual drug odors. Given the relative rarity of illegal drugs in cars stopped by police, even the best-trained dog is apt to be wrong far more often than right when he signals the presence of contraband. In light of these problems, which the Supreme Court has consistently ignored or minimized, marijuana legalization is merely the latest reason dogs should not be trusted to issue search warrants.