Dorset and Wiltshire Fire and Recuse Service were called to multiple heath fires near Wareham on the evening of Saturday, March 28.
DWFRS said crews were sent to the Sandford area following a series of incidents reported within a short period of time.
The first call was received at 6.52pm to a fire at Sandford Heath, covering an area of approximately 70 metres by 30 metres.
A second fire was reported at 7.06pm at Great Ovens Holl, followed by a third at Great Ovens Hill at 7.19pm.
Purbeck Police said the road closures were put in place due to a “large heath fire” near Lawsons Clump.
The B3075 Morden Road is closed between Forest Edge Rd and the A35 Morden Park Corner.
Around 60 firefighters were mobilised across the incidents, with crews using multiple small 4×4 appliances, heavy off-road vehicles, the Unimog from Wareham and water carriers.
The largest blaze was at Great Ovens Hill, where the fire covered an area of approximately 300 metres by 200 metres, with four separate areas affected.
A spokesperson for DWFRS said: “Firefighting operations continued for several hours, with crews remaining at the scene until around 1am on Saturday, March 29.
“The cause of the fires is under investigation but they are believed to be suspicious.
“Anyone with information is asked to contact Crimestoppers or the police.”