Officers are appealing for witnesses after a fire at the former American Adventure Theme Park.
Police and the fire service were called to the blaze at a construction site off Pit Lane, Shipley just after 10.30pm on Monday, April 8.
Three JCBs and one Caterpillar crawler loader were extensively damaged in the blaze.
Derbyshire Police said no one was injured in the incident.
A police spokesman said: "Officers are investigating the fire as arson and would like anyone with any information to come forward.
"Were you in the area at the time of the incident? Did you see anything suspicious?
"Anyone with information that may assist with our enquiries should get in touch using one of the following non-emergency contact methods.
"Please quote the reference number 19000180215 and the name of the officer in the case, DC Stuart Coutin, in any correspondence."
What to do if you need urgent police help but can't speak
Section of A1 closed overnight while crews tackled fire at industrial estate
Driving ban for man who drunkenly drove round and round a community centre car park
