Five deaths and one non-fatal heart attack are being linked to consumption of the highly caffeinated Monster Energy Drink, according to reports that the U.S. Food and Drug Administration (FDA) is investigating.

The reports claim that people had adverse reactions after they consumed Monster Energy Drink, which comes in 24-ounce cans and contain 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.

Monster Beverage Corp. has denied that its products caused any deaths, saying in a statement that "Monster is unaware of any fatality anywhere that has been caused by its drinks."

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

An autopsy concluded that Anais Fournier died of cardiac arrhythmia due to caffeine toxicity. The medical examiner also found that she had an inherited disorder that can weaken blood vessels.

The company touts Monster Energy Drink on its website as a "killer energy brew" and "the meanest energy supplement on the planet." The cans bear labels stating that the drinks are not recommended for children and people who are sensitive to caffeine.

Last year, sales volume for energy drinks rose by nearly 17 percent. The increased sales have brought heightened scrutiny from state and federal authorities.

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. 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.