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 Bill Bratton is urging the rank and file to refrain from making political statements.

“A hero’s funeral is about grieving, not grievance,” Bratton said in a memo to be read to all commands at roll calls on Saturday, the day Officer Wenjian Liu will be remembered at 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 TV monitors showing Mayor Bill de Blasio a week ago as he spoke at the funeral for the other officer who was killed, 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 who resisted arrest.

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.”

Investigators say Brinsley was an emotionally disturbed loner who started off his rampage by shooting and wounding an ex-girlfriend in Baltimore. He later killed himself.

The police killings ramped up emotions in the already-tense national debate over police conduct. Since Ramos and Liu were killed, police in New York have investigated at least 70 threats made against officers, and more than a dozen people have been arrested.

Cardinal Timothy Dolan and others have urged calm and asked all sides to tone down the rhetoric. Earlier this week, the mayor met with police union leaders.

Liu’s wake will be at the Aievoli Funeral Home in the Bensonhurst section of Brooklyn. His funeral is scheduled for Sunday with a Chinese ceremony led by Buddhist monks to be followed by a traditional police ceremony with eulogies led by a chaplain. Burial will follow at Cypress Hills Cemetery.

The 32-year-old officer had been on the police force seven years and had gotten married two months before he died. His widow, Pei Xia Chen, gave a tearful statement days after the shooting.

The funeral for Ramos was held last Saturday. Tens of thousands of officers and other mourners went to a church in Queens to pay their respects. Liu’s funeral arrangements were delayed so his relatives from China could get travel documents to the U.S. and fly to New York.

The Stephen Siller Tunnel to Towers Foundation, a charity created after 11 September 2001, has said it will pay off the home mortgages of the two slain officers.