Key Takeaways
- The Tenth Circuit’s ruling in United States v. Ostertag demonstrates that a drug dog’s alert can establish probable cause for a legal hemp vehicle search despite the similarity in smell between hemp and marijuana.
- Ostertag argued that the smell no longer indicates a crime, as both substances are chemically identical except for THC concentration.
- State courts are diverging from federal law, with some states like Michigan and Colorado limiting the use of odor alone as probable cause for searches.
- Legal ambiguity persists since hemp’s legalization in 2018 complicates traditional search and seizure law regarding cannabis.
- Drivers should be aware of their state’s laws, as some offer more protection against searches based on cannabis odor.
Hemp and marijuana come from the same plant. They look the same, they smell the same, and since Congress legalized hemp in 2018, only a lab test measuring delta-9 THC content can tell them apart. That single scientific detail has scrambled decades of search and seizure law that assumed the smell of cannabis was, by itself, evidence of a crime.
A new ruling from the Tenth Circuit Court of Appeals shows just how unsettled that law still is. In United States v. Ostertag, decided September 8th, the court sided with law enforcement even after the district court below it found that illegal marijuana and certain legal hemp smell “identical.” The case centers on a Wyoming traffic stop, a narcotics dog named Becky, and a legal argument that gets at the heart of what cannabis legalization was supposed to change.
This isn’t just a story about one traffic stop in Wyoming. It’s a window into how federal courts are handling a contradiction that legalization created but never resolved: if the smell of cannabis no longer reliably signals a crime, why does it still open the door to a search?
What Happened During the Wyoming Traffic Stop
George Ostertag was pulled over on Interstate 80 for failing to signal a turn. A K-9 officer arrived with Becky, a dog trained to detect marijuana, methamphetamine, and fentanyl. Becky first performed an open-air sniff around the exterior of the vehicle from the roadside. She was then brought closer, where she ran along the passenger side, jumped, and placed her paws on the car before settling into what her handler identified as alert behavior.
Officers searched the vehicle and found a 9mm pistol with an obliterated serial number. Ostertag, a convicted felon, was charged with unlawful possession of a firearm.
Before trial, Ostertag moved to suppress the gun, arguing that Becky’s sniff and her physical contact with the car both amounted to unconstitutional searches. He also challenged Becky’s reliability as a narcotics dog. The district court held a five-day suppression hearing and ultimately found that Becky’s alert behavior, which occurred before she touched the vehicle, was reliable enough to establish probable cause. Notably, the district court also found that unlawful marijuana smells identical to certain legal hemp. That finding became the foundation of Ostertag’s appeal.
Why Ostertag Argued the Sniff Itself Was a Search
Ostertag’s central argument challenged the legal assumption that has protected drug dog sniffs for two decades. He pointed to Illinois v. Caballes, the 2005 Supreme Court decision holding that a dog sniff during a lawful traffic stop isn’t a search at all, because it can only detect contraband and people have no legitimate privacy interest in illegal drugs.
Ostertag argued that logic no longer holds. Becky was trained to alert to marijuana, but marijuana and hemp are chemically identical apart from THC concentration, and only a lab test can distinguish them. If a dog’s alert can just as easily indicate a legal substance, Ostertag reasoned, then the sniff is no longer a narrow, contraband-only detection tool. It becomes something closer to general surveillance, capable of exposing lawful activity. He pushed the comparison further by citing Kyllo v. United States, a case involving thermal imaging of a home, to argue that any tool capable of revealing lawful conduct should count as a search requiring justification.
He raised a similar point about methamphetamine and fentanyl, both of which correspond to legally prescribed medications. If Becky’s alert on those substances could just as easily flag a prescription, he argued, the sniff risks exposing private medical information.
How the Tenth Circuit Rejected the Argument
The Tenth Circuit didn’t dispute the underlying fact that hemp and marijuana are indistinguishable by smell. Instead, it drew a distinction between what a sniff reveals and what officers later do with that information. According to the court, Becky’s alert disclosed only the presence of an odor. It didn’t identify the source of that odor, whether that source was legal or illegal, or what purpose it served. Determining any of that required additional police work: a visual inspection, a physical search, or further investigation.
The court described the leap from “odor detected” to “private information exposed” as several inferential steps removed from what the sniff itself accomplished. It leaned on Illinois v. Caballes and noted that a sniff remains limited by its own nature. It only picks up scents present at that moment, it doesn’t create a lasting record, and it can’t be used to prolong a stop beyond what the traffic violation requires. The court also rejected the comparison to thermal imaging, pointing out that cars carry a reduced expectation of privacy compared to homes, and that Ostertag hadn’t shown why the specific information at stake here deserved constitutional protection.
On the second issue, whether Becky’s paws touching the car turned the encounter into a search, the court sidestepped the question entirely. Even assuming that contact counted as a search, the court found that Becky had already alerted beforehand, which was enough to establish probable cause on its own. The vehicle search was upheld, and Ostertag’s conviction was affirmed.
The Contradiction Courts Haven’t Resolved
Here’s the tension at the core of this ruling. Congress legalized hemp nationwide in 2018. That means a substance sold openly in gas stations, wellness shops, and grocery stores across the country can trigger the exact same reaction from a trained narcotics dog as an illegal one. The Tenth Circuit acknowledged this reality. It didn’t pretend hemp and marijuana smell different. It simply concluded that the ambiguity doesn’t matter at the moment of the sniff, because the sniff itself doesn’t claim to identify anything beyond an odor.
Critics of this reasoning point out that it treats the sniff as legally meaningless while still allowing it to trigger real consequences: a search, a seizure, and in Ostertag’s case, a firearm conviction. If the odor is genuinely ambiguous between legal and illegal substances, the argument goes, then treating a dog’s alert as reliable evidence of “probable cause” starts to look more like a legal fiction than a factual finding. The Tenth Circuit’s answer is that probable cause has always required only a fair probability of criminal activity, not certainty, and that standard doesn’t demand the kind of precision hemp legalization would seem to require.
Where State Courts Are Taking a Different Path
While the Tenth Circuit held its ground, several state courts have moved in the opposite direction, often because their state constitutions or legislatures went further than federal law in protecting cannabis users. The Michigan Supreme Court ruled in People v. Armstrong that the smell of marijuana alone can no longer justify a warrantless vehicle search, given that state law permits adults 21 and older to possess and transport marijuana. The court held that officers now need something more than smell alone, like signs of intoxication or visible smoke, to establish probable cause.
Colorado’s Supreme Court reached a related conclusion in Colorado v. McKnight, ruling that a drug dog trained to alert on legal marijuana invades a legitimate privacy interest, meaning police need probable cause before deploying the dog at all rather than using the dog to establish it. Illinois, Minnesota, and Pennsylvania courts have adopted a middle position, treating marijuana odor as one factor among several rather than a standalone basis for a search.
Florida’s Second District Court of Appeal went furthest of all in October 2025, formally retiring the “plain smell doctrine” and holding that the odor of cannabis, without more, no longer establishes probable cause under the state’s updated cannabis laws.
Massachusetts courts have ruled that the smell of burnt marijuana can’t even establish reasonable suspicion, since the state decriminalized the drug. Arizona and Maryland went a step further and passed legislation explicitly barring police from stopping someone based on marijuana smell alone.
Tennessee’s Supreme Court took a position closer to the Tenth Circuit’s, finding that while hemp legalization makes a dog’s alert less conclusive, it can still factor into a probable cause determination.
Why the Federal and State Approaches Keep Diverging
The split comes down to a basic legal distinction. State courts that have limited the use of marijuana odor as probable cause are almost always interpreting their own state constitutions or statutes, which often provide broader privacy protections than the federal Fourth Amendment. Michigan’s ruling in Armstrong, for example, rested squarely on the state’s 2018 voter-approved law legalizing recreational marijuana. Florida’s ruling relied on the state legislature’s updated definition of cannabis. These are state-specific policy judgments layered on top of federal constitutional law, not replacements for it.
The Tenth Circuit, by contrast, was interpreting the federal Fourth Amendment, which doesn’t bend based on state cannabis policy. Federal hemp legalization changed what’s legal to possess, but it didn’t rewrite the constitutional test for what counts as a search.
That test, established in Illinois v. Caballes two decades ago, still asks whether a sniff invades a legitimate privacy interest, and the Tenth Circuit found that it doesn’t, regardless of how confusing the underlying chemistry has become.
This means the same fact pattern, a drug dog alerting on a car that might contain hemp or might contain marijuana, can produce opposite legal outcomes depending entirely on whether the case lands in state or federal court, and which state that happens to be.
What This Means If You’re Driving With Legal Hemp or Cannabis
For now, drivers in Tenth Circuit states, which include Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Utah, should understand that a drug dog’s exterior sniff of a vehicle during a lawful traffic stop is not, by itself, a Fourth Amendment violation, even where hemp and marijuana are legally and chemically difficult to distinguish. A trained dog’s alert can still form the basis of probable cause to search the vehicle under federal law.
That said, state law may offer more protection depending on where you live. Drivers and cannabis or hemp business owners in states like Michigan, Colorado, Florida, and Massachusetts have stronger footing to challenge a search based on odor alone. Keeping documentation such as receipts, lab results, or packaging that identifies a product as legal hemp remains a practical safeguard, since several courts have suggested this kind of evidence could weaken an officer’s claim of probable cause even where the sniff itself isn’t challenged.
The Legal Landscape Is Still Catching Up
The Ostertag ruling doesn’t settle the question about hemp, marijuana, and probable cause. It simply shows that federal courts are, for now, unwilling to let scientific ambiguity override a legal framework built before hemp legalization existed. Meanwhile, state courts and legislatures are actively rewriting that framework, one ruling and one statute at a time.
The gap between federal and state approaches isn’t likely to close soon, and it puts real weight on knowing your state’s specific laws around cannabis odor and vehicle searches. Given how quickly this area of law is shifting, anyone facing a search or charge tied to a drug dog alert should talk to an attorney familiar with cannabis law in their jurisdiction, since the outcome may hinge less on the smell in the car and more on which courthouse the case ends up in.
Frequently Asked Questions
Under the Tenth Circuit’s ruling in Ostertag, a trained narcotics dog’s alert can still establish probable cause for a vehicle search under federal law, even though the court acknowledged that legal hemp and illegal marijuana can smell identical. This applies specifically within the Tenth Circuit’s jurisdiction, which covers Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Utah.
No. Drug dogs are typically trained to alert to the scent of cannabis generally, not to distinguish between hemp and marijuana. Since the two substances differ only in their delta-9 THC concentration, a difference that isn’t detectable by smell, a dog’s alert cannot reliably indicate whether a vehicle contains a legal or illegal substance.
Yes, in some states. Michigan, Colorado, Florida, Illinois, Minnesota, and Pennsylvania courts have all ruled that the smell of marijuana alone is insufficient to establish probable cause, often citing state legalization laws or state constitutional protections that go beyond federal Fourth Amendment standards.
The 2018 Farm Bill legalized hemp at the federal level, but hemp and marijuana are the same plant species, differing only in THC content. Since courts have historically treated the smell of marijuana as evidence of criminal activity, hemp’s legal status undermines the assumption that cannabis odor reliably signals contraband, a contradiction courts are still working through on a case-by-case basis.
