Notes have been discovered at a sports pavilion which was damaged in an arson attack.
Pavilion A at Slades Farm, Bournemouth, is thought to have been torched on Sunday - the damage was discovered on Monday morning.
Notes left inside said the fire had been discovered and an extinguisher and buckets had been used to put it out.
The author wrote in the notes they did not contact emergency services for fear of getting into trouble.
Police said a fire exit door had been set alight and a 2ft-wide (60cm) hole burnt in the top of the door, which had recently been boarded up after previous damage.
Another door appeared to have been forced open where the notes had been left.
PC Joy Lee, of Dorset Police, said: "This property has been subjected to various incidents of criminal damage and anti-social behaviour recently but this incident is the most severe.
"It is concerning that the people who have seen the fire broke into the building and tackled the fire without calling the emergency services."
Anyone with information is urged to contact police.