 -- "A hero’s funeral is about grieving, not grievance."

That's the warning NYPD Commissioner Bill Bratton has sent to officers who plan on attending the funeral for slain officer Wenjian Liu on Sunday.

Hundreds Turn Their Back on de Blasio at NYPD Officer's Funeral

"Last Saturday, the New York City Police Department buried a hero. Tens of thousands of officers from our department and hundreds of other departments came to show respect for Detective First Grade Rafael Ramos. His family witnessed the kind of love that only his second family in blue could display. But for the last seven days, the city’s and the country’s consciousness of that funeral has focused on an act of disrespect shown by a portion of those tens of thousands of officers. It was not all the officers, and it was not disrespect directed at Detective Ramos, but all the officers were painted by it. And it stole the valor, honor, and attention that rightfully belonged to the memory of Detective Rafael Ramos’s life and sacrifice. That was not the intent, I know, but it was the result," Bratton wrote in a memo obtained by ABC News.

"On Sunday, we will gather together again, with the rest of New York City and law enforcement officials nationwide, to mourn for Detective First Grade Wenjian Liu. We gather to support his parents, his widow, and everyone who is there to remember a life tragically cut short. The assassination of Detectives Liu and Ramos was an attack on us all. As a cop, one who lived and worked through the assassination threats of the 1970s, I understand that emotions are high. I issue no mandates, and I make no threats of discipline, but I remind you that when you don the uniform of this department, you are bound by the tradition, honor and decency that go with it." Bratton added.

The officers were killed on Dec. 20 as they sat in their patrol car on a Brooklyn street. The shooter, Ismaaiyl Brinsley, later killed himself after posting on social media that he wanted to kill cops.

Police unions have blamed Mayor Bill de Blasio for creating a anti-police climate for allowing people to freely protest the recent grand jury decisions in Ferguson and Staten Island, something the mayor’s office has vehemently denied.