SAN FRANCISCO (KRON) -- Three former San Francisco mayors have signed into a letter calling for a criminal investigation into the current mayoral campaign of former interim mayor Mark Farrell. Art Agnos, Frank Jordan and Willie Brown have all signed onto the letter co-addressed to California Attorney General Rob Bonta and San Francisco District Attorney Brooke Jenkins. According to the letter, Farrell has violated election law and "funneled hundreds of thousands of dollars into a ballot measure to pay for expenses incurred exclusively by his campaign for mayor." Citing city campaign law that limits personal contributions to $500, the letter accuses Farrell of a scheme to use ballot measures to launder money. Michelin starred restaurant trashes SF’s SOMA in post announcing closure According to the letter, in a schemed uncovered by a San Francisco Chronicle investigation, Farrell "appears to be laundering significant sums of money through an unrestricted ballot measure committee to cover costs incurred by his campaign for mayor to circumvent the $500 per person limit." Quotes from Farrell's campaign cited by the Chronicle indicated that "pooled payments included costs for canvassers who were trained by Farrell's mayoral campaign staff to educate voters about the ballot measures." The canvassers, according to the letter, used Farrell's office space. But the newspaper reported that Farrell's campaign office had no presence and no staff on site for Proposition D. "Shared" payments for rent and payroll appeared to be intended to cover expenses incurred by Farrell's mayoral campaign to circumvent the $500 limit, the letter stated. Farrell's ballot measure committee, according to the letter, had subsidized 40% of his payroll and about half the rent for his campaign "under the guise of 'shared' expenses." DA_AG-Farrell-Investigation-LetterDownload "Curiously, through September 26th, campaign finance disclosures reveal no literature has been printed by the Proposition D campaign committee," the letter continued. "Since they have not printed literature to canvas for Proposition D, this suggests Proposition D money is subsidizing costs for staff working exclusively on Mark Farrell's campaign for mayor." The letter also accused Farrell of spending "significant sums" of Prop D money on campaign advertising. "While candidates can use ballot measure committees to pay for advertisements that have the effect of increasing their visibility with voters, those advertisements actually have to be about the ballot measure -- not the candidate's qualifications for office or public policy issues that have nothing to do with the underlying measure," the letter stated. The letter also cited Mission Local reports that Farrell's Prop D campaign spent hundreds of thousands of dollars on "advertisements that violate the law." Farrell, according to the letter, has repeatedly said that the activities outlined in the reports were vetted and approved by his attorney. But according to the letter, his attorney's memo doesn't "purport to authorize the conduct" that Farrell or his campaign engaged in. "Rather," the letter said, "it articulates the precise reasons why Mr. Farrell's conduct is clearly in violation of state and local laws." In addition to the three ex-mayors, the letter was signed by former City Attorney Louise H Renne, former Supervisor Angela Alioto, former State Senator Mark Leno, former Ethics Commissioner Quentin L. Copp, legal partner John Keker and criminal defense attorney Randy Know. According to the Chronicle, all three former mayors who signed off on the support different candidates for SF mayor, with Jordan supporting Daniel Lurie, Agnos supporting Aaron Peskin, and Brown supporting London Breed.