Authorities have seized nearly 20,000 marijuana plants from national forest land in Colorado this summer, part of an apparent expansion of growing operations funded and run by international drug cartels.

The operations pose a significant safety hazard to hikers who may happen upon the armed farmers in the woods. They also threaten streams that can be polluted by chemicals used to grow marijuana.

"I don't want it to get to the point where it is not safe for the public to go out into national forests," said Michael Skinner, assistant agent in charge of the U.S. Forest Service's Rocky Mountain Region.

In the latest discovery, in Pike National Forest near Deckers on Friday, dozens of federal and local agents found a rifle, piles of discarded garbage, propane tanks and more than 14,500 marijuana plants in an area the size of a football field. It could be the largest marijuana-growing operation ever found in Colorado.

Last month, authorities seized 5,100 plants, worth an estimated $2.5 million, from a pot-growing operation in Pike National Forest near Cheesman Reservoir.

Forest Service spokesman Terry McCann said rangers had previously encountered only small mom-and-pop marijuana-growing operations.

The bigger farms found this year indicate that well-funded drug cartels have discovered the Rocky Mountains, Skinner said.

"We don't know why they have decided to come here," he said. "This is new for Colorado. We haven't had time to study the trend."

Federal authorities arrested two suspects in the most recent case, both illegal immigrants from Mexico, Skinner said. He did not release their names or details about charges.

Skinner said he has requested $100,000 this year to cover costs for searching for the farms. He admitted it is a drop in the bucket against a growing problem.

Drug cartels, believed to be based in Mexico, are expanding into national forests to avoid the difficulty of moving marijuana grown in Mexico across the border. To maintain control, they import workers from that nation who spend months living in the forest monitoring the plants, authorities said.

But finding and catching them is difficult. Currently, 29 rangers oversee more than 14 million acres of forests and grasslands in Colorado. Each ranger must monitor an average of 500,000 acres each.

"That's what we're up against," he said.

In the Deckers operation, farmers suspected of being brought from Mexico carried miles of irrigation tubing, propane tanks and farm equipment to the site, about two miles east of Colorado 126.

The marijuana plants were sown on different days so that they could be harvested in stages, Skinner said, and the operators built a large shed where bundles of marijuana were hanging from the ceiling to dry.

The marijuana farmers used pesticides, weed killers and rat poison on crops within a few feet of Gunbarrel Creek. So chemicals could have drained into Gunbarrel, which empties into the South Platte River.

Workers stay close to the farms during the entire growing season, posing a safety risk to anyone hiking or hunting in the national forests.

Although no one in Colorado is known to have been attacked or killed while mistakenly walking into a marijuana growing camp, the potential is there, Skinner said.

He said people hiking in national forests need to be aware of several red flags, including trails formed where there shouldn't be any, as well as tortilla wrappers and beer, Spam and tuna cans left on them.

Also, people should not take the time to take pictures or record GPS positions. They should quickly hike away and call authorities, Skinner said.

Kirk Mitchell: 303-954-1206 or kmitchell@denverpost.com

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