The highly caffeinated Monster Energy Drink has been cited in five deaths and one non-fatal heart attack, according to reports that the U.S. Food and Drug Administration is investigating.

   The reports claim that people had adverse reactions after they consumed Monster Energy Drink, which comes in 24-ounce cans and contains 240 milligrams of caffeine, or seven times the amount of the caffeine in a 12-ounce cola.

   Although the FDA is investigating the allegations, which date back to 2004, the agency said the reports don't necessarily prove that the drinks caused the deaths or injuries.

   "As with any reports of a death or injury the agency receives, we take them very seriously and investigate diligently," Shelly Burgess, a FDA spokeswoman, said in a statement.

   News of the FDA's investigation follows a filing last week of a wrongful death suit in Riverside, Calif., by the parents of a 14-year-old girl who died after drinking two, 24-ounce Monster Energy Drinks in 24 hours.

An autopsy concluded that she died of cardiac arrhythmia due to caffeine toxicity and the medical examiner also found that she had an inherited disorder that can weaken blood vessels. But the child's parents claim Monster failed to warn about the risks of drinking its products.

   Monster Beverage Corp., which touts on its web site that the Monster Energy Drink is a "killer energy brew" and "the meanest energy supplement on the planet," puts labels on cans that state that the drinks are not recommended for children and people who are sensitive to caffeine. The company, based in Corona, Calif., did not immediately respond to calls seeking comment on Monday, but said last week that it is "unaware of any fatality anywhere that has been caused by its drinks."

   Monster is among a growing group of energy drinks on the market. Energy drinks are a tiny part of the carbonated soft drink market, representing about 3 percent of sales volume, according to a recent report by industry tracker Beverage Digest. But at a time when soda consumption is declining, energy drinks are becoming more popular: Last year, sales volume for energy drinks rose by nearly 17 percent.

   Monster has benefited the most from the increase. Last year, Monster had a 35 percent share of the energy-drink market based on volume, while Red Bull had 30 percent and Rockstar had 19 percent, according to Beverage Digest. Coca-Cola and PepsiCo are far smaller players in the arena, with about 5 percent each.

   Investors have warmed up to the drinks as well. In the past two years, Monster's shares have more than tripled, from about $22 and hit a high of about $79 in June. But on news of the FDA investigation, Monster's shares plunged $7.59, or 14.2 percent, to close at $45.73 in trading on Monday.

   The increase in popularity has brought heightened scrutiny. The levels of caffeine in the drinks have raised worries: Although the FDA caps the amount of caffeine in soda to 0.02 percent, there is no such limit for energy drinks.

   In August, New York state Attorney General Eric Schneiderman issued subpoenas to energy drink makers, including Monster, as part of the state's investigation of the industry. And in September, Senators Dick Durbin, D-Ill., and Richard Blumenthal, D-Conn., asked the FDA to take another look at the effect that caffeine and other ingredients in energy drinks have on children and adolescents.

The Associated Press contributed to this report.