The joint probe began last month just before a federal judge in California tossed out a lawsuit filed by Monster seeking to stop an investigation by San Francisco City Attorney Dennis Herrera.
Herrera has filed a lawsuit claiming the drinks pose health risks and accusing the Corona, Calif.-based Monster company of violating state law by misbranding its drinks and marketing them to minors. He began his investigation of Monster in 2012,
Meanwhile, New York Attorney General Eric Schneiderman has issued subpoenas to Monster and other energy-drink makers as part of his ongoing investigation.
Herrera said he believes the cooperative efforts between the two prosecutors' offices will prove beneficial for the public.
"We are disappointed that Monster has remained defiant in marketing products to children," Herrera said. "We hope this effort will cause the company to correct its irresponsible marketing practices."
Monster Beverage Corp. did not have an immediate comment on the partnership, spokeswoman Tammy Taylor said.
On its cans, Monster says the beverage is not recommended for children, people sensitive to caffeine, pregnant women or women who are nursing. Despite the warning, Herrera maintains the brand aggressively markets its products to kids.
Monster and other popular energy drinks have come under increasing scrutiny. The Food and Drug Administration has been investigating reports of deaths linked to energy drinks, but the agency noted that the reports don't prove the drinks caused the deaths.
Monster has repeatedly said its drinks are safe and it does not know of any fatalities caused by its products.
Schneiderman is also working with San Francisco District Attorney George Gascon and other law enforcement officials nationwide demanding that mobile phone manufacturers create kill switches to combat surging smartphone thefts across the country.