"It would a very challenging lawsuit to bring to preempt the state's decriminalization law," Cole told Senator Patrick Leahy (D-Vt.), who called the Senate Judiciary Committee hearing to address the conflict between state and federal marijuana possession laws.

Cole told senators about the legal reasoning behind the Justice Department's decision to forego a federal challenge to state decriminalization laws and instead focus on reviewing those states' regulatory schemes.

U.S. Justice Department officials decided not to challenge state laws that decriminalized marijuana because law enforcement officials feared government action could turn Colorado and Washington into black markets for the drug, Deputy Attorney General James Cole testified Tuesday.

"We might have an easier time with their regulatory scheme in preemption, but then what you’d have is legalized marijuana and no enforcement mechanism within the state to try and regulate it," Cole said. "And that's probably not a good situation to have."

That would mean an incentive for a black market, "and money going into organized criminal enterprises instead of going into state tax coffers and having the state regulate on a seed to sale basis what happens to it," Cole testified.

"We would be trying to overturn that, and yet there would still have decriminalization of marijuana so it would still exist in the state," Cole said.

Cole emphasized that the Justice Department explicitly reserved the right to challenge the states in court if federal officials don’t think the states' regulatory schemes are sufficient. "I think we're at a point now where we’re trying to find the best of the imperfect solutions in front of us," Cole said.