The deaths of two fans marred the weekend's Bonnaroo music festival in Tennessee, US, headlined by Bob Dylan.
Bonnaroo saw sweltering temperatures and thunderstorms
Tests showed both victims - the first in the festival's three-year history - had taken drugs, authorities said.
"The high humidity, high heat, alcohol and drugs - it's not a good combination," according to investigator Dale Brissey.
Other acts at the 90,000-capacity event included ex-Talking Heads singer David Byrne and rock band Wilco.
Festivalgoers endured sweltering sunshine and occasional thunderstorms to see more than 80 acts at the three-day event, held on a farm 60 miles (100 km) south of Nashville.
"When there is drug usage and the heat has been like it has, we have been expecting it every year," Coffee Country Sheriff Steve Graves said. "This year our luck ran out."
Police charged hundreds of people with drug offences, but handed out orders to appear in court rather than locking up suspected offenders.
"Our jail is too small to put that many in there at one time", Sheriff Graves said. He also said there was less violence than at previous festivals.
Many fans were treated for dehydration and heat exhaustion, according to US reports.
Fifty festivalgoers were sent to hospital by ambulance in one 12-hour shift, US newspaper The Tennessean reported.
And British singer Steve Winwood was a last-minute replacement for country singer Willie Nelson, who had to pull out because of carpal tunnel syndrome, a type of wrist injury.