The nation’s main advocacy group for medical marijuana threatened Tuesday to challenge Los Angeles City Atty. Carmen Trutanich’s legal assault on dispensaries, saying it is “unlawful, unconstitutional, and contravenes the spirit and letter of the governing laws.”

The city prosecutor’s office filed three lawsuits last week seeking court injunctions to force Organica in the Venice area and two Holistic Caregivers stores in South Los Angeles to stop all sales. Trutanich maintains that state law authorizes collectives only to grow marijuana and recover their actual costs, not to sell it.

Americans for Safe Access, which advocates for the use of medical marijuana and has defended dispensaries in court, has tried repeatedly to persuade the city prosecutor that he is misreading the law and recent court decisions, but he has not budged.

The organization’s chief counsel, Joseph D. Elford, sent letters to Trutanich and Los Angeles County Dist. Atty. Steve Cooley, who shares Trutanich’s view, saying the advocacy group would join the lawsuits unless they are withdrawn.

“We want to let Trutanich and Cooley know we’re not just going to sit this one out,” he said.

The organization’s decision increases the likelihood that Los Angeles, the ground zero of the state’s dizzying dispensary boom, also will become the center of litigation that clarifies murky issues.

Voters passed the state’s medical marijuana initiative in 1996 and the Legislature adopted a law to expand access in 2003, but the courts still have not ruled directly on whether collectives can sell marijuana to their members.

William W. Carter, the chief deputy city attorney, said he had not seen the letter.

“I’ve read their press release, and I am not impressed,” he said. “We obviously don’t agree with their position. We are enforcing the existing local and state laws just as we’ve been doing for a long time.”

Carter said the city attorney’s office had no intention of withdrawing the lawsuits, noting that it has already won an injunction barring one dispensary from selling marijuana in a similar case and is confident in its legal position.

Trutanich and Cooley have pressed cases that could force the courts to weigh in. Trutanich sued Hemp Factory V, an Eagle Rock dispensary, and his lawyers persuaded a Superior Court judge to take their side. Elford said he learned about the case too late to intervene before the decision.

Cooley’s office has filed felony charges against dispensary operators, saying their sales violate state laws.

On Monday, prosecutors charged Jeff Joseph, the operator of Organica, with 24 felonies.

Elford also accused the two prosecutors of taking preemptive action before the city’s medical marijuana ordinance takes effect. “They fought us tooth and nail with the City Council,” he said. “They’re not happy with the result.”

Trutanich and Cooley had pressed the council to explicitly ban sales, but lawmakers rebuffed them.

Pointing out that the city attorney’s office filed its lawsuit against Hemp Factory V months before the council approved the ordinance, Carter dismissed Elford’s contention, saying, “We were fully engaged.”

john.hoeffel@latimes.com