The 16-year-old died from a "caffeine-induced cardiac event causing a probable arrhythmia".
He had no pre-existing heart condition.
The teenager weighed 90kg (200 lbs) but would not have been considered morbidly obese, Mr Watts said.
"This is not a caffeine overdose," Mr Watts told Reuters news agency.
"We're not saying that it was the total amount of caffeine in the system, it was just the way that it was ingested over that short period of time, and the chugging of the energy drink at the end was what the issue was with the cardiac arrhythmia."
Caffeine would probably not have been seen as a factor in the teenager's death if witnesses had not been able to tell officials what he had been drinking before his death, the Richland County coroner said.
The main witness could not say which brand of energy drink Cripe drank but said it was from a container the size of a large soft drink.
"We're not trying to speak out totally against caffeine," Mr Watts said. "We believe people need to pay attention to their caffeine intake and how they do it, just as they do with alcohol or cigarettes."
In March, scientists at the Mayo Clinic said that up to 400mg of caffeine a day appeared to be "safe for most healthy adults".
But in 2015, the European Food Safety Authority said drinking more than 400mg could lead to increased heart rate, higher blood pressure, irregular heartbeat, tremors, nervousness, insomnia and panic attacks.
Cripe may have consumed about 470mg of caffeine in just under two hours, based on statistics from the website caffeineinformer.com.
It says a McDonald's latte has 142mg of caffeine, a 570ml (20oz) Mountain Dew has 90mg, and a 450ml (16oz) energy drink can have as much as 240mg.
More about: #Caffeine