Around 30 firefighters battled a blaze at a derelict building in Newtongrange overnight on 26 August.
The alarm was raised at 12.45am with a report of a building alight on Morris Road. Fire crews found the single storey building well alight on arrival.
The fire was finally extinguished using firefighters wearing breathing apparatus and a line of hose just before 3.30am.
A spokeswoman for Lothian and Borders Fire and Rescue Service said: “Fire investigation at the scene has now established that the cause of the fire was deliberate ignition. If anyone has information about this we would urge them to contact the police. Fireraising is a very serious matter which puts property and the safety of the public and the fire crews called out to deal with these fires at risk.”