A sign and a pin are pictured on the back of a demonstrator during a protest against the shooting death of teenager Michael Brown, in Ferguson, Missouri August 13, 2014. Police in Ferguson fired several rounds of tear gas to disperse protesters late on Wednesday, on the fourth night of demonstrations over the fatal shooting last weekend of an unarmed black teenager Brown, 18, by a police officer on Saturday after what police said was a struggle with a gun in a police car. A witness in the case told local media that Brown had raised his arms to police to show that he was unarmed before being killed. (Mario Anzuoni/Reuters)

The killing of 18-year-old Michael Brown has rightly provoked widespread outrage, drawing international media attention and prompting a comment from President Obama. The same should be true — but tragically is not — of the killing of 3-year-old Knijah Amore Bibb.

Brown was killed Saturday in Ferguson, Mo.; Knijah died the following day in Landover, Md. Both victims were African American. Both had their whole lives before them. The salient difference is that Brown was shot to death by a white police officer, according to witnesses, while the fugitive suspect in Bibb’s killing is a 25-year-old black man with a long criminal record.

I want to be clear: From what we know so far, the anger over Brown’s death is understandable and appears justified. Absent a full narrative from the police officer’s side, we are left with witness accounts alleging that the fatal encounter was triggered when Brown committed the unpardonable crime of “walking while black.”

We’ve been through this so many times. Brown, from all reports, was a good kid who had just graduated from high school and was about to enroll in college. But young black men are automatically assumed to be dangerous thugs — and are not given the benefit of the doubt that young white men are accorded. This is racist and wrong, and it must change.

But we should be just as outraged over Knijah’s death — and just as determined that this kind of killing should never happen again.

According to police, Knijah’s family was visiting friends at a house in Landover on Sunday afternoon. Among the people who lived at the address was a young woman whose boyfriend, Davon Antwan Wallace, had also dropped by.

Wallace got into a heated argument with the girlfriend’s teenage brother, police and family members told The Post. At issue was clothing that belonged to Wallace — and that the brother had apparently been wearing. Wallace allegedly left, went to his car, got a gun and fired about six shots at the second floor of the house, apparently aiming for the brother’s room.

One of those bullets struck Knijah and killed her.

“She liked to wear silver boots in the summer,” Knijah’s grandmother, Brenda Bibb, told The Post. “She had a Hello Kitty sticker on one boot and a Dora [the Explorer] on the other.”

The entire Prince George’s County police force — not just the homicide division — has been working long hours to try to find Wallace and is motivated by what a police spokesman called a “sense of moral outrage.”

That feeling should be universal. The near-constant background noise of black-on-black violence is too often ignored. Yet it continues to claim victims at a rate that our society should consider outrageous and unacceptable.

Landover is adjacent to Washington, where it has been a particularly bloody week: At least 21 people have been struck by gunfire since last Friday. Among them were an off-duty D.C. police detective who was shot in an attempted carjacking. Most of the victims fortunately do not have life-threatening injuries, but at least one was reported to be in grave condition, and one has died.

I’ve written about the sad customs that have developed in neighborhoods plagued by this senseless violence — the makeshift memorials of teddy bears and balloons, the speed with which T-shirts bearing a victim’s likeness are produced. This kind of death should never be thought of as ordinary.

The phrase “black-on-black violence” is more often used to distort rather than clarify. Crime depends largely on proximity and thus reflects patterns of racial segregation; the overwhelming majority of white murder victims are killed by whites, just as the overwhelming majority of black victims are killed by blacks. By the standards of most other developed countries, “white-on-white violence” in the United States is also of crisis proportions.

But it is disingenuous to pretend that a shocking disparity does not exist. According to FBI statistics, in 2012, the last year for which figures are available, 2,614 whites were killed by white offenders and 2,412 blacks were killed by black offenders — similar numbers. But the non-Hispanic white population is almost five times as large as the African American population, meaning the homicide rate in black communities is staggeringly higher.

Treating every young black man as a criminal — as may have happened to Michael Brown — is not the solution. We can understand the socioeconomic causes of violent crime without surrendering to them. We need to get angry — before we have to mourn the next Knijah Bibb.

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