WASHINGTON – The U.S. Department of Justice attempted to clear the air last week about how law enforcement will respond to state-based marijuana legalization efforts, saying the federal government will take a pass as long as states implement strict regulations.

The Aug. 29 announcement ended nearly a year of silence from Attorney General Eric Holder Jr., who was forced to weigh a conflict between state and federal law, which lists marijuana as one of the most dangerous drugs.

The Justice Department guidance came just three days after Congress stepped up pressure on Holder to decide whether to challenge Colorado and Washington state marijuana legalization laws. The Senate Judiciary Committee scheduled a Sept. 10 hearing to explore the issue, inviting Holder and Deputy Attorney General James Cole to testify.

"Our oversight on this issue was intended to provide movement on this policy question," Senator Patrick Leahy, D-Vt., the committee chairman, said in a formal statement in response to the Justice Department policy announcement. "All the more in a time when federal resources are especially scarce, the Justice Department should focus on countering and prosecuting violent crime, while respecting the will of the states whose people have voted to legalize small amounts of marijuana for personal and medical use."

Cole, in a memorandum to prosecutors, emphasized that the Justice Department still could challenge state laws if prosecutors find the states' regulatory systems do not effectively protect public safety and health.

Federal law enforcement priorities include preventing the diversion of marijuana from states where the drug is legal to those where it is not, Cole wrote. The government also will seek to prevent "state-authorized marijuana activity from being used as a cover or pretext for trafficking of other illegal drugs."

"A system adequate to that task must not only contain robust controls and procedures on paper; it must also be effective in practice," Cole wrote. State and local governments, he added, must provide "the necessary resources and demonstrate the willingness to enforce their laws and regulations in a manner that ensures they do not undermine federal enforcement priorities."

Marijuana Majority, a group opposed to the criminalization of marijuana, said that the Justice Department announcement "represents a step in a right direction." But Tom Angell, the group's chairman, was cautious about celebrating, saying the Obama administration has closed down more state-legal marijuana businesses in one term than the Bush administration did in two, despite a 2009 memo that said it was not a priority to bust medical marijuana providers operating in accordance with state law.

"It's significant that U.S. attorneys will no longer be able to use the size or profitability of a legal marijuana business to determine whether or not it should be a target for prosecution, but the guidelines seem to leave some leeway for the federal government to continue making it hard for state-legal marijuana providers to do business,"Angell said.

Ezekiel Edwards, director of the American Civil Liberties Union's Criminal Law Reform Project, called it "one more concrete step towards more sensible drug policy in this country."

New Jersey Governor Chris Christie told The Star-Ledger newspaper in Newark, N.J., that Holder's decision not to fight Colorado and Washington was a "mistake." Christie insisted "there's no such thing as medical marijuana, it's just marijuana."

For months, Holder delayed making any announcement. Testifying before the Senate in March, he said he would reveal a policy "relatively soon." He told a House committee in April that he would make a decision "as quickly as we can."

The Justice Department's extended silence left uncertainty about whether, for instance, a Coloradoan carrying a joint would face federal prosecution, or if Washington officials who license marijuana retailers risked prosecution for carrying out their duties.

The interplay between federal criminal law and the Washington and Colorado statutes raises tricky federalism concerns, said Covington & Burling special counsel Daniel Suleiman, who previously worked in the Criminal Division under Lanny Breuer.

"On the one hand, marijuana is a controlled substance, and possessing it is a federal crime. No two ways about that," Suleiman said. "On the other hand, the people in Washington and Colorado have spoken in favor of decriminalization, and the president has said we have 'bigger fish to fry.'"