DENVER (AP) — Authorities said Friday they raided hundreds of black market marijuana operations in Colorado that flouted the state’s cannabis law by growing tens of thousands of plants in Denver-area homes and selling the drugs out of state.

Investigators seized more than 80,000 plants and 4,500 pounds 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, U.S. Attorney Jason Dunn said.

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.