Seven Vietnamese have died and five are in a coma after taking drugs at an electronic dance music festival in Hanoi late Sunday, officials said, as authorities scrambled to trace the substance.
The victims, all Vietnamese, tested positive for an unspecified drug prompting officials to ban all EDM festivals in the capital in the wake of the fatal incident, which has shocked the conservative city.
"Police investigators are trying to figure out what type of drug that was," deputy director of Hanoi police Nguyen Van Vien told reporters Monday.
Officials said the dead were all in their 20s, while the five people in a coma are between 18 and 30 years old.
The sick are being treated in intensive care at two Hanoi hospitals.
If the deaths are confirmed as drug-related it could be one of the worst mass deaths at a music festival linked to narcotics in recent memory anywhere in the world.
Sunday night's "Trip To The Moon" festival was hosted by Vietnam Electronic Weekend in Hanoi's upscale West Lake, a neighbourhood popular among expats, teachers and youth for its nightlife and club scene.
Hundreds joined the event which sprawled across three stages and included EDM big hitters Yellow Claw and Headhunterz on the line-up, according to the online flyer for the event.
Investigators are trying to confirm who brought the drugs into the venue and said all events of its kind would be banned in the capital pending the probe.
"The incident yesterday... was very painful. It was a great loss," said Tran Xuan Ha, deputy head of Hanoi government's propaganda department said.
Tickets to the event cost between $21 and $135 in a country which has joined Asia's multi-million-dollar fixation on EDM.
Most ravers are found in the cosmopolitan and more free-wheeling southern hub of Ho Chi Minh City.
But the EDM party scene has gained popularity in the capital of the communist nation, as has drug use among young people.
It was the third Vietnam Electronic Weekend festival held in the country and was slated to be the biggest yet, according to state media.
Shocked Hanoians took to the internet Monday after the deaths.
"A warning message for those young ones who have been so irresponsible for their own (lives)," reader Nam Huynh wrote on VNExpress.
There are some 220,000 registered drug users in Vietnam, according to official figures.
In the past, most users were hooked on opium and heroin, though strong synthetics such as ICE and E are becoming increasingly popular, especially among Vietnam's partying youth.
The synthetic substances largely come into Vietnam through porous borders from the notorious 'Golden Triangle' drug producing zone -- comprised of lawless parts of Laos, Myanmar and Thailand -- though some high-profile busts in recent years in Vietnam have uncovered domestic labs.
On Saturday evening, two Australians died of suspected drug overdoses at an electronic music festival in Sydney, according to national broadcaster ABC.