According to the investigation, an electrical wire leading into the battery area of the vehicle short-circuited, causing the fire that spread to the electrical system and other vehicles on the ground floor of the building.
Meanwhile, three of the four fire extinguishers at the site remained unused, the investigation said.
The fire broke out at 11.50 pm on September 12 from the ground floor of the mini apartment building on Lane 29/70, Khuong Ha Street. The 200-square-metre 10-storey building housed 150 people who lived in 45 mini apartments.
The fire was controlled in over 45 minutes but left 56 dead and 37 injured. Over 90 survivors escaped the flames.
The owner of the building, Nghiem Quang Minh, 44, was arrested and prosecuted on September 13 for violating fire safety regulations. He will be detained for four months.
Investigations showed that Minh was given a construction permit by Thanh Xuan District authorities in 2015 to build a 167-square-metre six-story private house. However, instead, he expanded the project and built a 10-storey building on an area of 230 square metres.