Thousands of police officers and ordinary citizens came out on Sunday to mourn the death of Wenjian Liu, a police officer who was shot along with partner Rafael Ramos on Dec. 20, 2014.

Wenjian Liu, a son of Chinese immigrants who came to this country when he was 12-years old, was killed along with his partner by a mentally unstable man named Ismaaiyl Brinsley. The man later killed himself in a subway station as police were closing in on him.

Brinsley had a history of violence and mental instability. Before attacking the police officers, he had shot and injured his girlfriend and made multiple threatening online references to the killings of unarmed black men at the hands of white police officers that took place this past summer.

Speakers lined up to pay tribute at the service at Aievoli Funeral Home funeral home.

Police officers could be seen turning their backs to Mayor Bill de Blasio's video screen address at the funeral.

According to The Associated Press, de Blasio – who has garnered disdain from many members of the rank-and-file New York Police Force for his apparent sympathy for anti-police protesters this winter – said to the congregated mourners, "Let us rededicate ourselves to those great New York traditions of mutual understanding and living in harmony. Let us move forward by strengthening the bonds that unite us, and let us work together to attain peace."

Prior to the funeral, NYPD Commissioner William Bratton asked police officers not to turn their backs on the mayor, as it would draw attention from the proceedings, according to USA Today.

Wenjian Liu’s open casket funeral was delayed so his family members could come from China.

Rafael Ramos's funeral was held on Dec. 27, 2014. More than 20,000 officers attended, many turning their backs.

New York police commissioner Bill Bratton urged officers via a memo to refrain from turning their backs: "A hero's funeral is about grieving, not grievance."

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