A 20-year-old Philadelphia woman was pronounced dead shortly after being found unconscious at a campsite at the annual Firefly Music Festival in Dover, Del., on Sunday morning.

Dover police identified the woman as Caroline Friedman and said the cause of her death was still under investigation, although it was not the result of foul play.

Officers found Friedman on Firefly Camping Lot 18 around 6:30 a.m. She was pronounced dead at Bayhealth Kent General Hospital at 6:55 a.m., police said.

It was the first death in the music festival's seven-year-history, said Master Cpl. Mark Hoffman, a spokesman for the Dover Police Department, though last year, police found human remains near the event's campgrounds during the festival.

In 2017, organizers said 90,000 people came out to Firefly, a four-day event that calls itself the biggest festival of its kind on the East Coast, to camp and see live music. Eminem and Kendrick Lamar were among this year's headliners.

Deaths at outdoor music festivals are not uncommon: At Bonnaroo in Manchester, Tenn., one of the most popular music and camping festivals in the country, more than a dozen people have died in its 17-year history, including a 35-year-old man at the festival June 8. In 2016, Los Angeles County passed laws designed to keep festival attendees safe, not long after the death of two young women at a music festival in Pomona, Calif. A nonprofit called DanceSafe, which promotes safety in the electronic dance community, warned that drugs being sold at parties are not often what they seem: Out of more than 500 samples of the drug MDMA, or "Molly," MDMA was present in only 60 percent of the samples they tested.

In 2016 and 2017, police arrested nine people at Firefly for selling drugs ranging from MDMA to LSD to cocaine.