Ed Andrieski / AP A federal agent enters VIP Cannabis dispensary in Denver on Thursday, Nov. 21, 2013 during a raid on the business.

Federal agents raided an unknown number of marijuana dispensaries and growing sites in Colorado Thursday, confiscating piles of marijuana plants and cartons of cannabis-infused drinks just weeks before the state allows recreational marijuana retailers to open their doors.

The raids, conducted on a snowy morning, were the first in Colorado since the U.S. Department of Justice said in August that it wouldn't interfere with state marijuana laws as long as the states keep the drug away from children, the black market and federal property.

The U.S. Attorney's Office in Denver said the federal action "comports with the Department's recent guidance" but would not elaborate. The U.S. Attorney's Office said authorities were executing sealed search and seizure warrants and wouldn't disclose how many businesses are being targeted or what they're being investigated for.

In a statement, spokesman Jeff Dorschner said the Internal Revenue Service's criminal investigations division was also involved.

Retail marijuana sales are to begin Jan. 1 in Colorado, though not all municipalities will be ready to go by then. For now, dispensaries are supposed to sell only to people with medical permission to use the drug. Many of the state's 500 or so existing dispensaries are making plans to convert to recreational sales.

The Justice Department said in August that it wouldn't stand in the way of votes in Colorado and Washington to legalize recreational pot but warned there needed to be effective controls to keep marijuana away from children, the black market and federal property.

At one of the raided dispensaries, VIP Cannabis, agents took boxes out of the business and loaded them into a U-Haul truck. One officer wore a surgical mask.

In Boulder, agents raided a number of marijuana-growing warehouses, leaving a chest-high pile of marijuana plants on the side of a road before loading them into trucks, The Daily Camera reported.

Colorado's marijuana industry was quick to point out that the raids did not necessarily mean that the federal government was going back on its word or planning to interfere with Colorado's plans to a recreational market.

Mike Elliott, a spokesman for the Medical Marijuana Industry Group, said the industry itself has pushed for "robust" regulations and expects industry compliance with lengthy state regulations on how the drug is grown and sold.

"While everyone involved in these raids should be considered innocent until proven guilty, enforcement is a sign that this program is working and maturing," he said in a statement.

Mason Tvert of the Marijuana Policy Project said it wasn't clear how many businesses were raided. Many dispensaries and growing warehouses were operating as usual Thursday.

"The Justice Department said it would respect states' rights to regulate marijuana, and that it would not go after businesses as long as they are complying with state laws," he said in a statement. "We hope they are sticking to their word and not interfering with any state-regulated, law-abiding businesses."