Police are now offering a $25,000 reward in relation to the police car that was set alight on Friday morning.
The car was parked on the forecourt of Mandurah Toyota, located on Anstruther Road near the intersection of Pinjarra Road in Mandurah.
About 4.30am, two people approached the dealership on foot and poured an accelerant over the car, before igniting it with a butane torch.
A police spokesperson said the police car was extensively damaged by the fire and was not repairable.
"It is believed one of the offenders may have suffered burn injuries to their face and arms," the spokesperson said.
Arson Squad investigators urge anyone who has mobile phone or dash-cam vision of the area between 4am and 5am, or who has information relating to this fire, to call Crime Stoppers on 1800 333 000 or report online at www.crimestopperswa.com.au.
Rewards of up to $25,000 are available for information that leads to the identification and conviction of an arsonist.
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