In October 2009, medical marijuana advocates celebrated a U.S. Department of Justice memo declaring that federal authorities wouldn't target the legal use of medicinal pot in states where it is permitted.

The memo from Deputy U.S. Attorney General David Ogden was credited with accelerating a California medical marijuana boom, including a proliferation of dispensaries that now handle more than $1 billion in pot transactions.

But last month brought a new memo from another deputy attorney general, James Cole. And this time, it is stirring industry fears of federal raids on pot dispensaries and sweeping crackdowns on large-scale medical pot cultivation.

Cole asserted in the June 29 memo that state laws "are not a defense" from federal prosecution, saying, "Congress has determined that marijuana is a dangerous drug" – and that distributing it "is a serious crime."

Justice Department officials said the memo offered "guidance" for states permitting medical marijuana and didn't mark a harsher shift in federal policy. But it was a clear signal of the government's concern about a move toward industrial-scale operations that would generate millions of dollars in revenue.

The memo came off as a threat to Steve DeAngelo, director of the Harborside Health Center, California's largest medical marijuana provider. He charged that President Barack Obama and Attorney General Eric Holder are turning their backs on medical users and imperiling the distribution of marijuana as medicine.

"I can't imagine why the Obama administration wants patients to obtain their medicine from a criminal market rather from a licensed and regulated system of distribution," said DeAngelo, whose Oakland dispensary has 50,000 clients and handles more than $22 million in annual transactions. "I just can't imagine them following through on their position."

Advocates and legal observers are split on whether the concern expressed in the Cole memo over the "scope of commercial cultivation and use of marijuana for purported medical purposes" signals raids on pot stores. But many say it is a backlash against cities and entrepreneurs trying to cash in on the medicinal pot trade.

The city of Oakland is still exploring a plan to regulate and tax expansive marijuana cultivation, despite warnings by federal authorities that they wouldn't tolerate earlier city efforts to sanction cavernous indoor marijuana farms.

Recently, federal authorities indicted two Sutter County tomato farmers who were growing marijuana for an Oakland businessman who claimed he was providing medical pot to dispensaries.

And the threat of federal prosecution killed the ambitions of the Sacramento Delta town of Isleton to reap a tax windfall from an entrepreneur's plan to grow medical pot in multiple greenhouses.

"I think the federal government never anticipated what really has happened," said Michael Vitiello, a McGeorge School of Law professor who has studied efforts to legalize pot. "And the government is saying, 'Wait a second.' "

In February, U.S. Attorney Melinda Haag in San Francisco declared that the Justice Department was "considering civil and criminal remedies" against anyone trying to set up "industrial marijuana-growing warehouses in Oakland." The Alameda County district attorney warned that meant public officials weren't immune from prosecution.

Oakland City Councilwoman Patricia Kernighan said the city hasn't given up on taxing and licensing medical marijuana cultivation. But she said the council, due to vote Tuesday on doubling the number of local dispensaries from four to eight, will wait until fall before deciding on a scaled-down proposal to let each dispensary operate marijuana growing rooms only for its own registered patients.