The pioneering musician and Who cofounder was found dead in his room at Las Vegas' Hard Rock Hotel on June 27--the day before the band was to kick off a three-month tour.
"The heart attack we believe was brought on by the significant amount of cocaine" that was in Entwistle's system at the time of death, coroner Ron Flud told the Associated Press.
Flud says it's not possible to quantify the amount of cocaine Entwistle had consumed.
"Cocaine is a different animal," Flud explained. "It's not like alcohol. There's no way we can put a number on it. You've got a lethal drug on board at the time you have a bad heart. That's a bad combination."
Flud said Entwistle's coronary arteries had already been damaged by heart disease. The cocaine exacerbated the condition and triggered the fatal heart attack.
At the time of Entwistle's death, Steve Luongo, the drummer and comanager of the John Entwistle Band, confirmed the bassist had been taking medication for a heart condition.
There was no comment Thursday from either the Who or the Hard Rock. Authorities say no criminal charges will be filed.
Known for his stoic on-stage demeanor and fiery fret-work, Entwistle was regarded as one of rock's all-time greatest bassists. His signature songs included "My Wife" and "Boris the Spider."
Entwistle is the second member of the Hall of Fame band to succumb to a drug-related death. Drummer Keith Moon overdosed on anti-alcoholism pills in 1978 at age 32.
The band's surviving two members, Pete Townshend and Roger Daltrey, decided to press on with the tour as a tribute to their fallen friend. Their next concert is scheduled for Friday in Mansfield, Massachusetts.