Outrage spread across social media Monday when a user posted a photo of a Portland police car with a blue and white "Blue Lives Matter" sticker affixed to the back.

That's a violation of City of Portland policy. According to rules about visual markings for city vehicles, no "private decals or markings are allowed," on police vehicles.

Lieutenant Tina Jones, a Portland Police Bureau spokesperson, says PPB is "appreciative this has been brought to our attention and will continue to utilize our systems of accountability to ensure we perform within the highest professional standards."

She declined to comment on "what discipline would be imposed for a minor policy violation." She says "it depends upon a variety of factors, such as whether someone has had prior command counseling (which is not discipline) for a similar matter, or if someone has prior discipline it could result in unpaid time loss, for example."

She adds that the agency doesn't know who placed the sticker on the car.

"There is no 'officer in question' for several reasons," she says. "First, we share vehicles so we can be good stewards of the public money, which means officers are not assigned one vehicle for patrol. Second, we do not know who put the sticker on the vehicle, including whether or not it was an officer."

The Blue Lives Matter flag grew popular as a response to the Black Lives Matter movement. Some view it as support for police officers. Others see it as a racist attempt to belittle protests against police brutality.

Jones says PPB has "notified all staff to check for unauthorized stickers or markings and to notify a supervisor if something is found in violation of the policy."