Investigators said the fire started in a red Range Rover Sport which arrived at the car park at 20:42 BST on the night of the fire.
CCTV footage showed it entering the building with light coloured smoke coming from its engine.
At 20:47 the driver continued to the third floor when he stopped the car, exited, and called 999.
When first fire crews arrived at about 20:55 there were multiple cars involved in the fire, estimated to be about three visible from outside the car park.
At 21:07 the group commander arrived and was briefed that at least six cars were ablaze.
The fire is described in the document as “one of the more significant incidents to occur within Bedfordshire Fire & Rescue Service’s area for many years”.
