Nine people aged between 17 and 29 died of suffocation in the northern city of Hai Phong on Sunday after inhaling exhaust fumes from a car, police said.
The autopsy report showed that all of the victims, including two women, had used drugs, they added.
Pham Van Thuc, father of one of the victims, said he tried to reach his son on the phone but couldn't.
So he went looking for his son at his house on Nguyen Van Hoi Street at around 3 p.m., only to find seven dead bodies on the floor and two others who were almost dead.
Those two people were rushed to the hospital but pronounced dead a short time later.
Police said after the group left a night club on Saturday night, they came to Thuc's house to dance and consumed ecstasy tablets there.
At around 1 a.m. on Sunday morning, there was a power cut. The group then brought a car inside the house, using the car lights and its music player to party on. As they left the engine turned on for a long time, they were suffocated by the exhaust fumes.