Earlier this week, Justice Department spokeswoman Nanda Chitre said enforcement of the federal Controlled Substances Act remained unchanged.
"In enacting the Controlled Substances Act, Congress determined that marijuana is a Schedule I controlled substance," Chitre said. "We are reviewing the ballot initiative and have no additional comment at this time."
Eric Brown, a spokesman for Hickenlooper, would not say whether the governor planned to disclose the details of his call with Holder.
If Colorado's marijuana ballot measure is not blocked, it would take effect by Jan. 5, the deadline for the governor to add the amendment to the state constitution. The measure allows adults to possess up to an ounce of marijuana, and six marijuana plants, though public use of the drug and driving while intoxicated are prohibited.
Colorado's measure also directs lawmakers to write regulations on how pot can be sold, with commercial sales possible by 2014.
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