The head of emergency services in Buenos Aires, Alberto Crescenti, said doctors were still identifying the exact cause of death of the festival attendees. Most of the victims were in their early 20s.

"We have to wait for their autopsies to determine what mix of substances [was responsible]," said Crescenti.

Initial tests have indicated that the victims took the psychoactive drug known as Ecstasy, hospital sources told local daily "Clarin."

Several of the surviving patients have slipped into a coma with very high fevers, a sign of severe poisoning, according to Crescenti.

"They are all in very grave condition, intubated," Crescenti told the Todo Noticias station. "They are not breathing on their own."

Designer drugs in a bus

The victims had been attending the Time Warp international electronic music festival, which hosted some 11,000 visitors in a crowded Buenos Aires venue.

A number of people in the crowd "were offering drugs, pills, acid," according to festivalgoer Ignacio.

"They tried to sell me drugs four times," he told C5N TV.

Police also found a large quantity of designer drugs while searching a bus headed to the event. Officials seized Ecstasy pills, LSD, marijuana, poppers and cocaine, and arrested two people in the vehicle.

Second night canceled

Event organizers have offered their condolences over the deaths.

"The cause of this tragedy will take some time to determine," they said on the festival website. "The show is cancelled for tonight."

Time Warp originated in Germany in the mid-1990s and has since spread to several cities across the world. It has been held in Buenos Aires since 2013.

dj/cmk (AFP, AP, Reuters)