NEW YORK — As thousands of mourners prepare to attend the wake and funeral of a second New York City police officer killed in an ambush shooting, police Commissioner William Bratton is urging the rank-and-file to refrain from making political statements.

“A hero’s funeral is about grieving, not grievance,” Bratton says in a memo to be read to all commands at roll calls on Saturday, the day Officer Wenjian Liu will be remembered during a wake. “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’s comments referred to hundreds of officers who turned their backs to giant TV monitors displaying the remarks of Mayor Bill de Blasio a week ago at the funeral for the other slain officer, Rafael Ramos. That gesture mimicked one made by police union officials outside a hospital two weeks ago when the officers were killed.

Union officials, who are negotiating a contract with the city, have said the gestures were made out of frustration with the mayor. They say de Blasio helped foster an anti-police atmosphere by supporting demonstrations following the chokehold death of an unarmed black man on Staten Island by a white officer.

Liu and Ramos were ambushed sitting in their patrol car on a Brooklyn street by Ismaaiyl Brinsley. Brinsley had made references online to the killings of unarmed black men at the hands of white police officers, vowing to put “wings on pigs.”