Police were on Monday combing a tavern where 21 teenagers mysteriously died as survivors described a battle to escape the jam-packed premises and one reported a suffocating smell.
Officials have ruled out a stampede as the cause of the deaths.
Most of the victims, some as young as 13, were found dead inside Enyobeni tavern in East London.
Seventeen died inside the bar, while four died in hospital. The victims included 13 boys and eight girls.
A prayer service was held for 21 teens who died. Video: Bheki Radebe
Thirty-one others were hospitalised with symptoms including backache, tight chests, vomiting and headache, officials said.
Most were discharged on Sunday, leaving two in hospital, they said.
The fatalities bore no visible signs of injury, sparking initial speculation among local officials and politicians that this was a case of under-age drinking that went tragically wrong.
"But the suspicion is that it is something either they ingested through drinks, food, or something they inhaled," Unathi Binqose, a government official on safety, told AFP.
New details also emerged Monday as survivors spoke of a strong and suffocating smell in the jam-packed double-storey building.
A member of staff at the bar, Sifiso Promise Matinise, told AFP he sprinkled water on the unconscious people to revive them, thinking they were drunk, before realising what had happened.
"I saw two people collapse, they died," he said.
Sinovuyo Monyane, 19, who was hired by the bar to promote an alcohol brand, said she was still "confused" but felt lucky to be alive.
She said she struggled to escape through a door gridlocked with people.
"We tried moving through the crowd, shouting 'please let us through,' and others were shouting 'we are dying, guys,' and 'we are suffocating' and 'there are people who can't breathe'," she told AFP.
"I passed out at that moment. I was running out of breath and there was a strong smell of some type of spray on in the air. We thought it was pepper spray," she said.
She later regained consciousness after someone sprayed water on her.
"I got up and realised that there were bodies lying around. I saw people being poured water, but those people did not even move," she said in a phone interview.
"I could have died."
Special investigators from Pretoria have been rushed to the scene but no arrests have been made so far.
"The investigators continue to search for possible clues and answers at Enyobeni Tavern," regional police spokesman Thembinkosi Kinana said.
Many of the victims are thought to have been students celebrating the end of their high-school exams, officials said.
Autopsies are being conducted to see if the deaths could be linked to poisoning.
"Post-mortems (were) completed by last night and the bodies will be released to their families today," said Yonela Dekeda, provincial spokeswoman for the health department.
Forensic analysis will be conducted this week.
"Samples were taken and were on (the) first flight today to Cape Town, where the tests will be conducted," said Binqose.
President Cyril Ramaphosa is among those who have voiced concern.
The teenagers reportedly "gathered at a venue which, on the face of it, should be off-limits to persons under the age of 18", he said.
A resident DJ, Luhlemela Ulana, who was also celebrating his birthday on the night, spoke of a rush of revellers who forced their way into an already packed venue.
"We tried to close the door but people kept pushing. The bouncers could not handle the crowd that was pushing from outside the entrance door. There were so many people," the DJ said.
He turned off the music to try to discourage the revellers, but to no avail.
The crowd was just "unruly and could not be managed", he said, adding he was "traumatised".
About 100 mourners attended an emotional prayer service at the Assemblies of God church in the Scenery Park township, where local municipal councillor Monica Goci broke down on the pulpit with a microphone in her hand.