AP photo.

Federal DEA and IRS agents conducted raids today at medical marijuana businesses in Colorado, where voters recently approved the legalization of pot for recreational use.

A federal official tells ABC News the raids in the Denver area, which also included state and local police, involved fewer than 15 businesses and grow operations, and that no arrests were made Thursday.

The Daily Camera newspaper published photos and video of tractors loading large piles of marijuana plants into dump trucks at a facility near Boulder, Colo.

A spokesman for the U.S. Attorney in Denver would not say what officials were looking for or why the businesses were targeted, citing the ongoing investigation.

"The Drug Enforcement Administration, Internal Revenue Service Criminal Investigations, the Denver Police Department and state and local law enforcement are today executing lawfully obtained search warrants and seizure warrants," U.S. attorney spokesman Jeff Dorchner said in an email to ABC News.

"Although we cannot at this time discuss the substance of this pending investigation, the operation under way today comports with the department's recent guidance regarding marijuana enforcement matters," Dorchner said.

That "recent guidance" is contained in this August memo, issued to all federal prosecutors from Deputy Attorney General James Cole. The memo says that federal authorities will not stand in the way of legalized marijuana in Colorado or Washington state, but will enforce these eight priorities it says are "particularly important" to the feds:

"Preventing distribution to minors; Preventing revenue from marijuana sales from going to criminal enterprises, gangs, and cartels; Preventing diversion of marijuana from states where it is legal to other states; Preventing state-authorized marijuana activity from being used as a cover or pretext for the trafficking of other illegal drugs or other illegal activity; Preventing violence and the use of firearms in the cultivation and distribution of marijuana; Preventing drugged driving and the exacerbation of other adverse public health consequences associated with marijuana use; Preventing the growing of marijuana on public lands and the attendant public safety and environmental dangers posed by marijuana production on public lands; Preventing marijuana possession on federal property," the memo says.

"There are strong indications that more than one of the eight federal prosecution priorities identified in the Department of Justice's August guidance memo are potentially implicated," Dorchner said.

On January 1, Colorado becomes the first state to allow legal recreational marijuana sales in retail stores to adults. Washington state voters also approved legal marijuana in 2012, and retail sales there will begin this spring.