LOS ANGELES (AP) -- Federal prosecutors looking to take out California's medical marijuana shops have now set their sights on Los Angeles, where city officials have struggled to stop a blooming of dispensaries.

The U.S. attorney's office sued three property owners that house pot collectives and sent warning letters to 68 others as they enforce a federal law that doesn't recognize a California initiative that legalized pot for medicinal use.

The move Tuesday came nearly a year after federal authorities began targeting the state's pot shops. The city's own ban on dispensaries also is being challenged and could be overturned by voters if a referendum is placed on an upcoming ballot.

"As today's operations make clear, the sale and distribution of marijuana violates federal law, and we intend to enforce the law," U.S. Attorney Andre Birotte Jr. said.

California's four U.S. attorneys pledged last October to curb pot collectives they said were running afoul of the law by raking in huge sums of money and serving as fronts for drug traffickers. Proponents argue the dispensaries are protected by California law that allows medicinal use of marijuana with a doctor's recommendation.

David Welch, an attorney representing some of the Los Angeles collectives targeted by federal authorities, said he plans to file a lawsuit.

"I expected this to happen and we have planned for this contingency," Welch said. "The future is a lot less certain considering what seems to be a full press by the federal government."

Los Angeles passed an ordinance two years ago that was supposed to shutter hundreds of pot dispensaries while capping the number in operation at 70.

But a set of legal challenges against the city by collectives and the recent expiration of the ordinance due to a sundowner clause led to another surge of pot shops. City officials said more than 750 collectives have registered with the city and as many as 200 more could exist.

City officials have had a difficult time striking a balance between providing safe and affordable access to pot for people who need it for medicinal purposes while addressing neighborhood groups' worries that streets were being overrun by dispensaries and pot users.

"The shops had an opportunity to work with the city on a path to legitimacy, but once again they chose short-term profits over long-term safe access for legitimate patients," said Michael Larsen, president of the Eagle Rock Neighborhood Council.

More than 175 California cities and 20 counties have banned retail pot shops, according to the medical marijuana advocacy group Americans for Safe Access.

The state Supreme Court is expected to address whether local governments can ban medical marijuana clinics, but a hearing hasn't been set by the high court.

Los Angeles could soon face a possible referendum on the latest ban approved by the city council. Council members must decide by next week whether to call a special election for the measure, repeal it themselves or put it on the March 2013 ballot.

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