As far as marijuana grow operations go, the three warehouses in southwest Denver certainly looked legit.

They were in areas where commercial cultivation operations are common. They had wireless security cameras, build-out plans drawn up by an architecture firm and contracts with a carbon-dioxide supplier. They even employed a security company often hired by legal marijuana businesses.

But, according to search warrants filed in Denver District Court, they lacked one key component: a license.

A worker at one of the grows nabbed by police told detectives, “he did not know if it was licensed, but was pretty sure it was not legal,” according to a search warrant affidavit.

When police raided the warehouses in February and March, they took nearly 1,300 plants.

But Denver authorities have yet to file charges against anyone connected to the warehouses, and a police spokesman said the case is ongoing. Far from an old-fashioned slam-bang marijuana bust, the case has come to show how difficult police say it now is to investigate suspected illegal marijuana growing in Colorado.

“We’re seeing some trends toward placing illegal grows near legal grows,” Denver police spokesman Sonny Jackson said. “But every case is different. … Just because a grow is unlicensed doesn’t mean it’s illegal.”

Since the passage of limited marijuana legalization in Colorado in 2012, law enforcement officials say many agencies are making fewer busts of illegal marijuana distributors and seizing fewer illegally grown plants, though comprehensive numbers aren’t yet available.

But those same officials contend the drop-off comes not from a decline in illegal growing but from an increased hesitance of detectives to make busts in a state where the margins of legal and illegal cultivation can be blurry. What’s more, if police seize and destroy marijuana later deemed to be legal, they could be sued for damages.

“People are unsure of what they can and can’t do,” Tom Gorman, the director of the Rocky Mountain High Intensity Drug Trafficking Area, said of investigators. “People are shying away. There’s a lot of confusion out there by law enforcement.”

Adults in Colorado are allowed to grow up to six marijuana plants in their homes or other enclosed spaces. Medical marijuana patients and caregivers, however, may be able to grow more. Commercial grow operations need a license, but that can be the subject of prolonged administrative processes.

Search warrants detailing what raids there have been show that police believe some suspected illegal growers are trying to capitalize on the confusion.

The three warehouses in southwest Denver — on South Jason and South Kalamath streets — are an example.

Also this year, Denver police raided another suspected illegal grow in a warehouse district popular with legal growers, as well as a suspected illegal marijuana distribution operation that advertised itself with its own website.

The Denver district attorney’s office has charged three people in connection with a hash-oil explosion at an unlicensed warehouse in the city.

Jim Gerhardt, a sergeant with the North Metro Drug Task Force in Adams and Broomfield counties, said most of the unlicensed grows detectives have busted in his area are in homes. But he said the task force’s number of busts “are way down” this year, something he attributed to pot legalization.

When anyone over 21 can grow up to six plants, it’s tougher for police to unravel whether the plants they find are legal or not.

“There’s a ton of grow operations out there,” Gerhardt said. “There’s a ton of activity in the community. It’s just not as easy to investigate as it was.”

Marijuana advocates, however, say police claims of investigative hardship are overstated.

Mason Tvert, one of legalization’s most visible proponents in Colorado, said the basic police work behind marijuana cases today remains unchanged: Figure out what’s going on and apply it to the law.

“They were counting marijuana plants and determining what the penalty should be before,” Tvert said. “And that’s the same as it is today. … It’s really difficult to see why this is so confusing.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold