The U.S. Food and Drug Administration says it’s investigating reports of five deaths and a non-fatal heart attack linked to highly caffeinated Monster Energy Drinks.

The agency acknowledged the adverse reports Monday, but FDA spokeswoman Shelly Burgess says they don’t prove that the drinks caused the deaths.

The news follows last week’s filing in California of a wrongful death suit by the parents of a 14-year-old, Hagerstown, Md., girl who died after drinking two, 24-ounce Monster Beverage Corp. drinks in 24 hours.

An autopsy concluded she died of cardiac arrhythmia due to caffeine toxicity. She had an inherited disorder that can weaken blood vessels.

Monster says it doesn’t believe its products caused any deaths.

Shares of the Corona, Calif.-based company plunged $7.20, or 13.5 percent, to $46.12 in trading Monday.