Elise Schmlzer reports for the Denver Post on People v. McKnight, 2019 CO 36. Here is an excerpt from her article:

The Colorado Supreme Court greatly diminished the role of police dogs trained to detect marijuana with a ruling Monday that created another divide between how state and federal law enforcement can investigate pot.

In a 4-3 ruling, the Colorado Supreme Court ruled that, under the state constitution, a dog trained to alert to marijuana cannot be used before an officer establishes probable cause that a crime had been committed.

For decades, police dogs were trained to alert their handlers to the presence of pot. But since Coloradans voted in 2012 to legalize recreational possession of small amounts of the drug, the dogs’ sniff tests have been controversial because they can alert even if a person has a legal amount of marijuana.

Monday’s ruling effectively renders the dogs trained to detect pot useless in most situations, said Sam Kamin, a law professor at the University of Denver who studies marijuana law and policy. Previously, the dogs’ sniff tests were used to create probable cause for a search. Now, there has to be enough evidence to authorize a search before a marijuana-trained dog can be used, making the dogs’ sniff tests redundant.