The windows of the petrol-bombed police car were attacked while an officer was still inside the vehicle.
A police officer was also waiting outside the vehicle. Both officers escaped unhurt.
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A report published by the Boundary Commission for England details plans to reduce the number of MPs by scrapping 50 constituencies.
The slump will spark scrutiny, especially as statistics show a rise in racially or religiously motivated offences following the EU vote.
The 53-year-old was killed shortly after leaving her home in Mosta, Prime Minister Joseph Muscat said.