The outgoing U.S. attorney general said he is "cautiously optimistic" when it comes to Washington and Colorado's experiment with marijuana legalization.

Eric Holder, who announced last month his plans to retire, is one of President Barack Obama's longest-serving Cabinet members and has faced the delicate task of defining federal policy after a wave of marijuana legalization at the state level across the country.

"We don't want to put into the federal system, low level people who are simply there for possessory offenses," Holder said Monday in an interview with CNN's Evan Perez

Holder last year outlined eight enforcement areas the Justice Department would focus on in a move aimed at calming nerves in Washington and Colorado, the only two states where recreational marijuana is legal.

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The eight "priority areas" have focused the Justice Department's efforts on preventing marijuana distribution to minors, inter-state trafficking and drug violence.

But Holder made clear Monday that his agency could change its non-interventionist stance if the states' regulatory frameworks aren't up to snuff.

"What I've told the governors of those states is that if we're not satisfied with their regulatory scheme that we reserve the right to come in and to sue them. So we'll see," Holder said.

The sale and possession of marijuana remains illegal under federal law.

The Justice Department's policy hasn't meant total immunity for marijuana growers and dispensaries in states with both medical and recreation marijuana, where dispensaries and growhouses have since been raided and owners prosecuted.