A number of cars were deliberately torched outside HMP Kilmarnock on Friday night.
Police said six vehicles were “extensively damaged”, but no one was injured in the incident.
The Scottish Fire and Rescue Service (SFRS) mobilised two appliances to the East Ayrshire prison after the alarm was raised at around 7.19pm.
A spokesperson for SFRS said fire crews left the scene at around 9.14pm.
Investigating officers are checking CCTV and enquiries so far have revealed that a dark-coloured car was seen driving away from the prison after the motors were set on fire.
Detective inspector Ewan Bell said: “I’m sure someone will have seen or noticed this dark-coloured car as it sped off from the prison car park.
“All of the cars involved have been extensively damaged and I am appealing to people who were in the area around the time of the incident to contact us.
“Also, I would appeal to any motorists to check their dashcam footage as it may have captured an image which could assist us in our enquiries.”
Craig Thomson, Serco prison director at HMP Kilmarnock, added: “Last night six cars were set on fire in the prison car park.
“No one was hurt.
“We are working closely with the police to help them find the people who carried out this attack and have provided them with CCTV footage.”
If you have any information, call 101 or Crimestoppers anonymously on 0800 555 111.