Colorado and federal agents have raided illegal marijuana operations across the state the past six weeks in a concerted effort to take out syndicates marketing marijuana to different parts of the country.

U.S. Attorney John Walsh’s staff has filed numerous criminal charges in recent weeks against illegal operations.

“This wave of marijuana grow operation sites is greater than we’ve seen the last couple of years,” Walsh said. “They seem to have the view that ‘if I come to Colorado and set up a marijuana grow operation, I won’t be noticed.’ “

Walsh speculated that the influx could be linked to the drought in California, where many illegal grow operations have been found previously. Colorado is more desirable because it is not in drought, he said.

“Marijuana takes a lot of water to grow,” Walsh said.

Local, state and federal agents have arrested 34 people, few of them Colorado residents, on illegal marijuana charges the past six weeks. Some of the offenders were citizens of Mexico, Honduras and Cuba. About 20,000 marijuana plants were destroyed as well as 300 kilograms of dried marijuana.

The search for illegal grow operations will continue to be aggressive, he said. At the time that Colorado passed its recreational law, federal agents warned that they would be particularly vigilant about illegal operations that exported marijuana to other states.

In every case in which arrests were made, the marijuana grows were both illegal under federal law and unlicensed by state authorities under Colorado’s marijuana regulatory system, authorities said.

• The biggest illegal marijuana ring that was dismantled was in southern Colorado. Authorities arrested 20 people on Sept. 1 for their alleged roles in a marijuana syndicate that harvested pot at eight properties — more than 1,000 plants — and shipped it to Florida via UPS. Agents confiscated 28 guns.

• Agents destroyed an illegal grow operation with 3,900 marijuana plants on Aug. 19 in the Pike National Forest in the Green Mountain Area in Jefferson County. The agents confiscated 3,000 pounds of irrigation pipe, pesticides, flammable liquids, camping gear and trash.

• Two Mexican nationals were arrested Aug. 28 in the Buffalo Pass area northeast of Steamboat Springs. Agents confiscated a gun and destroyed 1,000 plants.

• Two men were arrested Sept. 7 in the San Isabel National Forest near Cordova Pass. Agents destroyed 11,700 plants and confiscated a rifle, irrigation pipe, pesticides, flammable liquids, camping gear and trash.

• Federal agents arrested four Mexican nationals on Sept. 15 near a grow operation along the Dolores River on Bureau of Land Management land. BLM rangers found more than 1,200 marijuana plants up to six feet tall and confiscated 211 kilograms of dried marijuana and a rifle.

• BLM agents also found 1,000 marijuana plants on Sept. 30 in Montrose, also near the Dolores River. Rangers arrested one Honduran and five Mexican nationals at or near the site.

Kirk Mitchell: 303-954-1206, kmitchell@denverpost.com or @kirkmitchell or denverpost.com/coldcases