Investigators seized more than 80,000 plants and 4,500 pounds (2,040 kilograms) of harvested marijuana, state and federal prosecutors said at a news conference. Officers raided 247 homes and eight businesses and arrested 42 people in Denver and seven nearby counties.
State law allows up to 12 marijuana plants per residence for personal use, but some of the homes had more than 1,000 and many had hundreds, said U.S. Attorney Jason Dunn.
Colorado and nine other states have broadly legalized marijuana use but it remains illegal under federal law. That has created tension between some state and federal officials.
But George Brauchler, district attorney for the south and east Denver suburbs, stressed the investigation was a joint state-federal operation, not the U.S. Department of Justice imposing its will on Colorado.
"Make no mistake, we are equal partners in this," Brauchler said.
State and federal officials said the nearly three-year investigation showed that illegal marijuana trafficking mushroomed after voters approved recreational use in 2012.
Dunn said Colorado has become the epicenter for a nationwide black market in marijuana.
Brauchler warned that Colorado is becoming "the wild West of weed." He said the provision in the law that allows small-scale home marijuana cultivation opened the door to big, illegal operations.
Brauchler warned that other states considering allowing home marijuana plants could expect the same but added he was not trying to discourage them from doing so.
"I think states are entitled to do whatever they want," he said. "But they need to know the reality of this."
Mason Tvert, a spokesman for the Marijuana Policy Project, which advocates for decriminalizing marijuana, questioned how prosecutors know that Colorado's law attracted illegal growers.
"Did they conduct a survey of illegal marijuana cultivators to determine why they decided to operate where they did?" he said. "Are they able to know whether those operations existed prior to legalization or not?"
Tvert blamed the illegal operations on states that still ban marijuana, and said if they legalized and regulated it as Colorado does, there would be little illegal production.
Dunn said investigators plan to use federal forfeiture laws to seize 41 homes, 25 vehicles and $2.2 million in cash connected to the marijuana operations.
He said the 41 homes have an average market value of $400,000.
"These grow operations are not occurring in abandoned houses or poor parts of the metro area," he said. "These are happening in middle- and upper-class neighborhoods where many of us live and raise families, and they're occurring all over the metro area."
William McDermott of U.S. Drug Enforcement Agency said the Colorado operations did not appear to be violent. He said officers seized a few guns but would not say how many.
Sixteen of the suspects were arrested on federal drug charges and 26 on state charges.
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