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DENVER -- The Denver Police Department said Colorado’s illegal marijuana business is thriving.

"The black market marijuana is booming," Cmdr. James Henning said.

Last year, Denver police arrested 242 people for illegally growing, selling or extracting marijuana. Henning's team seized 8,913 pounds of marijuana last year.

“That’s driven simply by the old laws of supply and demand. People are buying marijuana for a low price and buying low and selling it high," he said.

Local police work with the Drug Enforcement Administration to eradicate illegal grows across the state, including several outdoor cultivation sites in Pueblo, Mesa, El Paso and Garfield counties.

Pictures released by the DEA and police show hundreds of potted, pruned and THC-producing plants confiscated on the black market. Police say the illegal business is not only booming, it’s increasingly more dangerous.

“We are finding more weapons, They are a little edgier. We know that in that black market, there’s a lot of ripoffs and robberies going on, but nobody reports it to us because you don’t report that you are robbed while doing an illegal activity," Henning said.

Law enforcement said the illegal market is flooded with high-quality Colorado medicinal marijuana. Red card users can buy 2 ounces a day for $100 to $150. Users can turn around and sell an ounce for $350 to $400.

Statistics from state patrol agencies across the country show Colorado marijuana mainly goes to seven states, with 65 percent of the weed coming from Denver.

In just three years, law enforcement across the country have seized about 4.5 tons of marijuana from Colorado.

Ads on Craigslist promise “safe dealings” and “overnight delivery” to out-of-state buyers. Police say it’s all buying and selling on the black market.

“We also have many local investigations anywhere from your small Craigslist operation where a guy is on Craigslist saying or offering you marijuana right now or I can ship marijuana anywhere in the United States. It’s all 100 percent illegal,” said Henning.

Users said Colorado’s marijuana market might have stopped low-quality weed from coming into the state, but it has opened the door for millions worth of top quality illegal weed to be sold tax free outside the state.