Alcohol, fatigue, domestic violence or mechanical failures could have all caused a 4WD to swerve into a road train in the Northern Territory, killing the six passengers including up to four children.
The vehicle was on fire for more than five hours after colliding with the truck near a small town in the Katherine region on Friday afternoon.
Very little human remains could be found once the fire was put out, though police believe four of the six passengers killed were children.
NT Police Commissioner Michael Murphy said investigations were ongoing.
"We haven't ruled anything out, we're looking at fatigue, distraction, the involvement of alcohol," he said on Tuesday.
"While we call it an accident, it's not really an accident
"There's some sort of element involved that could contribute to that whether it's mechanical, it could be road surface, it could be roadway, human."
Mr Murphy said domestic violence would also not be ruled out.
The driver of the road train and a passenger believed to be sleeping were taken to hospital with burns and broken bones but have since been released.
The road train was carrying mangoes and used cars and had just refuelled with 10,000 litres in Darwin when the 4WD swerved into its lane
Mr Murphy said the first responders and the general public who witnessed the crash would be feeling the psychological effects.
"(This is) a very traumatic and distressing and upsetting situation and a loss of young life too," he said.
Detective Senior Sergeant Brendan Lindner told reporters on Monday it would be a long process to identify the victims.
"The intensity of the fire meant that there was little human remains left," he said.
Forensic teams are using DNA analysis and are not expected to complete their work until Thursday.
The collision is the deadliest territory road crash in more than 15 years and police say the psychological impact will be "phenomenal".
"There's nothing to suggest the truck driver has done anything wrong, in fact, his actions at this stage sound commendable," Sgt Lindner said.
Police have received dashcam footage from people in the area at the time which is helping determine the cause of the crash.
They believe the family was from the NT's Big Rivers region and the 4WD had been bought the previous week.
Police reopened the Stuart Highway on Sunday after significant delays in the area over the weekend.
There have been 19 lives lost on NT roads this year.
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