A police car parked in Halifax was damaged when stones were thrown through the front window.
Officers were dealing with routine inquiries and had parked their car on Westbury Place, off Hopwood Lane, when it was attacked by yobs at 3.30pm on Tuesday.
They say inquiries as to how this happened are continuing.
The car was not a marked police car and two other vehicles belonging to members of the public were also damaged in the same area at around the same time.
Those two attacks happened at 4.10pm on Hopwood Lane and 4.45pm on Marsh Way.
Officers want to speak to four males described as Asian and aged between 15 and 18 who were seen in the area where the third car was damaged.
Any witnesses to any of the incidents is asked to contact Calderdale Police via 101.