New York City Police Commissioner William Bratton urged officers on Friday not to make political statements and show respect when Mayor Bill de Blasio speaks at the funeral of Officer Wenjian Liu, who was gunned in the borough of Brooklyn along with another officer in December. Thousands are expected to attend Liu’s wake on Saturday and his funeral on Sunday.

Bratton addressed the officers through a memo distributed to the city's police officers amid tensions between de Blasio’s administration and police department over the mayor’s controversial comments on the protest between law enforcement officials and civilians. 

"A hero's funeral is about grieving, not grievance," Bratton wrote. "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 was reportedly referring to officers who turned their backs when de Blasio spoke last week at the funeral of Officer Rafael Ramas, who was also shot and killed.

Bratton reportedly described the behavior of hundreds of officers turning around to giant TV screens showing de Blasio’s speech as an “act of disrespect,” adding that although every officer didn’t do it, “all the officers were painted by it.

“It stole the valor, honor, and attention that rightfully belonged to the memory of Det. Rafael Ramos’s life and sacrifice,” Bratton wrote, according to The Wall Street Journal, using the officer’s posthumous rank. “That was not the intent, I know, but it was the result.”

Liu and Ramos were shot by Ismaaiyl Brinsley while they were sitting in a patrol car on a Brooklyn street on Dec. 20. Brinsley, who committed suicide soon after killing the two officers, had previously said on social media that he would kill cops to avenge the deaths of Michael Brown and Eric Garner, who were black, unarmed and killed by white police officers.

The deaths of the two African-Americans sparked angry protests that intensified when grand juries in both cases decided not to indict the officers.

Garner was killed after NYPD Officer Daniel Pantaleo put him in a chokehold on a Staten Island street in New York in July, while Brown was fatally shot by Officer Darren Wilson in August in Ferguson, Missouri.

Police union officials had blamed the mayor for fueling anti-police sentiments with his comments over the Staten Island grand jury decision.

Liu's wake is scheduled to take place at the Aievoli Funeral Home in the Bensonhurst section of Brooklyn. His funeral will be conducted with a Chinese ceremony followed by a traditional police ceremony. Liu’s burial will take place at Cypress Hills Cemetery, The Associated Press reported.