Video footage has captured the fire which broke out and damaged a number of cars outside HMP Nottingham at the weekend.
Police were called to the prison, in Perry Road, Sherwood, in the early hours of Saturday, May 11.
Officers, who received reports that cars had been damaged and set on fire, said no one was injured.
They clarified there was no risk to the prison building and added they were investigating CCTV footage.
A video of the fire was captured by Shaun Byrne.
Speaking previously, a Prison Service spokesman added: “The fire service successfully resolved an incident in the grounds of HMP Nottingham early this morning.
“While no one was harmed, the safety and security of our staff is our priority and we are working closely with the police to urgently investigate what happened."
A Nottinghamshire Fire and Rescue Service spokesman said: "Crews from Stockhill and Arnold were called at 12am on Saturday, May 11, to the scene of a fire in a prison car park on Perry Road, Sherwood.
"Four hose reels were used to extinguish the blaze. Three cars were severely damaged by fire.
"No one was injured.
"Firefighters had dealt with the incident by 1.08am.
"An investigation into the cause is still ongoing."
Anyone with information which may help the police investigation is asked to call Nottinghamshire Police on 101, quoting incident 25 of May 11.
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