San Francisco City Attorney Dennis Herrera has filed a lawsuit claiming the drink poses a health risk, and accused the Corona-based company of violating state law by misbranding its drink and marketing them to minors.
Meanwhile, New York Attorney General Eric Schneiderman has also issued subpoenas to Monster and other energy-drink makers as part of his investigation.
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 Herrera. He now 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," said Herrera. "We hope this effort will cause the company to correct its irresponsible marking practices."
The FDA has been investigating reports of energy drink-related deaths.
The parents of 14-year-old Anais Fournier filed a wrongful death lawsuit against the company last year. The girl had pre-existing health problems, but her parents claim her death was caused by the two 24-ounce Monster Energy Drinks she had consumed within a 24-hour period.
According to the Mayco Clinic, two 24-ounce Monster Energy drinks contain 480 milligrams of caffeine. That's the equivalent of about five eight-ounce cups of coffee, or about 14 cans of cola.
While the FDA regulates the amount of caffeine in soft drinks, it does not with energy drinks, which are sold as dietary supplements.
Monster has repeatedly said its drinks are safe. The company released the following statement Wednesday: "The sale and consumption of more than 10 billion Monster energy drinks worldwide over more than 11 years has shown that our products are safe. Contrary to allegations, they are not "highly caffeinated" and they are not marketed to children. In fact, a 16-ounce Monster Energy drink contains less than half the caffeine of a 16-oz (medium) size cup of Starbucks brewed coffee. Monster's labels state: 'Consume responsibly: Not recommended for children, people sensitive to caffeine, pregnant women or women who are nursing.'"